What if James had family Harry could live with?
I imagine James would have a slightly younger brother named David or something (Fleamont gives his kids very basic names). David would have had a very strong brother complex as he and James were growing up, just because James is/was naturally excellent at damn well everything. So he was always extremely jealous of James, especially since I'm imagining they look very similar so they were always compared in that regard as well. I don't think Fleamont and Euphemia favored James, even unconsciously. They would have been equally, if not MORE thrilled to have a second kid as they were to have the first, and each of the boys was wicked spoiled and pampered growing up. David just became aware of James' talent for sort of everything, natural charisma, role as the heir of the family, and every other thing he had going for him and got jealous and pessimistic, seeing the situation as being "I don't have a place in the scheme of things."
He didn't go dark or anything stupid like that, he just went sorta bitter in the way of underperforming younger siblings. He always worked to outdo James, especially in personal life stuff, so he got married very shortly after James, or maybe even slightly before (they'd have to be very close in age for this to work, but that's fine) and his oldest daughter is only a few weeks younger than Harry, so they're in the same year at Hogwarts. Then he has a bunch more kids after that one because he's really invested in having a son to prove that he is able to continue the Potter line just as well as James did, but they all turn out to be girls and his wife, Carrie, puts her foot down before the 7th daughter's birth and that's just the way the family is. They have seven since they were working on the Weasley model where the 7th kid, Ginny, broke the mold, but in their case it didn't work.
So Harry grows up with them after Voldemort's attack instead of the Dursleys, and is mostly happy, though his uncle is sometimes sort of Snape-y in assuming Harry is just like James and sort of transferring his resentment to him. Just like a tiny bit, nowhere near as much as Snape does, but still, that wears on a kid. I don't know what this would bring to the table besides just a different starting point for Harry, but it would be cool if there were some twist we could have on the actual plot. So that's something to think about. What changes with Harry knowing about magic, his parents, Voldemort, etc, right from the start? Does it change how he deals with Voldemort when he comes back? How do his relationships with his cousins shape him as he grows up, since those would be largely positive as I imagine it? Would this be a story that really dug into family lore and old family/Sacred 28/Ministry politics? Is this a fic with all kinds of family magic involved? Does David resist/resent having to teach it to Harry since he still hopes to have a son of his own whom he could finagle into taking the Potter seat? Or maybe even a lot of it was lost with James since some things only get taught to the heir and obviously Harry was too young to learn any before James died? That would be interesting. Also, the narrative he would have grown up with from David would probably not be the "Your parents were martyrs and heroes and amazing and wonderful" so even though he still gets a lot of hero worship from the public, and everyone else he interacts with basically feels that way about Lily and James, he may actually feel sort of conflicted about them, almost like because they 'irresponsibly' decided to stand up to Voldemort, it's their fault that they died and left Harry an orphan. "What did they expect to happen?" David can say. I figure Sirius would still have gone to Azkaban, Peter's still ratified with the Weasleys, and Remus is off doing whatever Remus does till third year, so Harry doesn't have a strong presence giving him an alternative story.
Maybe David reacted to James' marrying a Muggle-born and going all maverick with the Order of the Phoenix by becoming a very staunch traditionalist, so he's very proud of how old the Potter family is and that his branch, at least, is still pureblood. He's not quite a pureblood supremacist in the way the Malfoys are, but he does draw his own self-value from that aspect of his identity. A sort of "There's nothing wrong with Muggle-borns, but I surely wouldn't want any of my daughters marrying one" attitude. More slimy and insidious than out and out bigoted. That Harry is half-blood and destined to carry on the Potter name is a real thorn in his side. Maybe later on he's always encouraging him to date pureblood girls (maybe I want to make it a Harry/Hermione fic for just that reason? Or maybe a Harry/Fleur? Then you've got all kinds of interesting racism to muck around with, like I can see David saying "For Merlin's sake, boy, she's hardly even a real witch!" Or maybe Harry/Gabrielle to avoid the weird age difference. And I would feel bad about messing up Bill and Fleur, they were cute. But wait, how old was Gabrielle? Hmm, he's 3 years younger than Fleur and five years older than Gabrielle. Weird either way. We could just age her up a year or four? Anyway.).
It would be interesting to explore David's opinions on the Potters not being on the formal Sacred Twenty-Eight list, and maybe he's eager to marry a bunch of his daughters into those families (gross). Haha, maybe he's gunning for his oldest daughter and Draco to marry someday. Maybe his wife is even from one of those families? Maybe a Fawley or a Selwyn or a Greengrass? Hmm, so are David's daughters cousins with Daphne and Astoria? I like it. I also think it would be cool to figure out how Dorea Black and Charlus Potter figure into things here, and, if they have descendants, what they would be like and how they're politically important. Does David view their son and whatever descendants he has as a rivals to the Potter legacy? HMM.
Family tree:
-Fleamont is deceased, would have been 84 or so when the story starts
-Euphemia is deceased, would have been around the same age, presumably (or maybe younger? With this math, she'd be 52 when James is born and 54 for David. Not impossible I suppose, but I might take her down a notch to, say, 77 or -8 instead.)
-James is deceased, would have been 32 when the story starts
-Lily is deceased, would also have been 32
-David is 30 or 31 when the story starts
-Carrie is 30 or 31 when the story starts
-Harry is 11 when the story starts
-Layla is 11 when the story starts (her name is just a nice basic girl's name since David is sure the next one will be a boy so he doesn't have to worry)
-Katherine is 8 when the story starts (name is more formal-two daughters is nothing to panic over, but he wants to be taken seriously. Maybe it's Carrie's mother's name? Or like a famous female Minister for Magic or something? Though according to PotterWiki, the recent female MfMs have been named Millicent (she's actually the one in office during the First War, so that might look a bit like sucking up, naming her after the sitting Minister, but whatever), Eugenia, and Wilhelmina. Any of them would work, though Eugenia and Euphemia are pretty similar.)
-Euphemia is 6 (starting to worry here, so he names her for his mother, who was a very proper pureblood woman whom everyone respected. Layla, Harry, and sometimes Carrie and Katherine call her Mia, which makes David furious)
-Iolanthe is 4 (named her for the woman who brought the legendary Peverells into the Potter line, showing how seriously he takes his name and legacy)
-Viviane - 2 (from the Arthurian legends where Nimue/Vivian/Nimiane was a sorceress powerful enough to subdue Merlin. Morgan or Morgana would also work here and get more name recognition, but I think Viviane sounds fancier, and Morgan/Morgana is a little too similar to Mallory imo)
-Rowena - 1 (obviously named for Ravenclaw, the more impressive (by David's estimation) female founder of Hogwarts. I think she should eventually go to Hufflepuff, just because)
-Mallory is 0 when the story starts (Carrie told him before the birth that this would be their last child no matter what, so when she turned out to be a girl he chose a name that meant 'unlucky' and 'unfortunate'. Real freaking nice.)
As for how this applies to the actual plot of defeating Voldemort and whatnot, I see it going one (or hell, both) of two ways: either it's all about secret weird subtle magic he has to learn over the years till he has something really formidable to bring to bear against Voldy in the final showdown, or it's a more political angle where he knows how to use his clout in the Ministry better than he does in canon (though perhaps it's more accurate to say 'at all'...) and make sure Voldy never gets as much of a foot up as he does in canon. But I also think there should be an aspect of Harry working against all kinds of prejudice in the world, partially by living that truth by getting with Gabrielle or Fleur or Hermione but also by going up very visibly in the Ministry and standing up for rights of all kinds of disenfranchised people, from Muggle-borns to werewolves to house elves (hell, maybe he actually takes SPEW seriously, and that could be something he and Hermione bond over if that's the ship that's endgame—or even if not, I mean, friends still need stuff in common), and I think ultimately he should campaign against having family magic at all and say that everything should be publicly taught and widely known so that the wizarding community can move forward all the better. I think his friendship with Ron should spark this idea, since not only is Ron not the oldest son, but I don't even think Arthur should be the oldest of his brothers, so all of his talented children are deprived of learning this really cool, interesting magic that could really help them out. I bet Layla has some stuff to say about this too, since as a daughter, she wouldn't be taught anything anyway. David is incensed by all of this, naturally, but I think that's half the fun of this premise. So there's still Voldemort stuff happening, but it's also very tied up with themes of equality and fairness and equality of opportunity, with a big dollop of family drama on top.