Here's some Fairy Tail OCs/Inserts. The hardest part is making them relevant to endgame when dragon slayers are the only useful wizards.
Dragon Slayers: I could make any Dragon Slayer OP.
SI: Natsu
Combine Yamamoto from Bleach and Madara from Naruto. Enough said.
SI: Gajeel
Gajeel's non dragon slayer form of iron magic can be some type of Idle Transfiguration from JJK. He can actually form a dragon of iron around himself to use his dragon slayer spells. He can also spit out molten iron / liquid iron of some such. Also iron sand is worth considering.
Non dragon slayers:
SI: Gildart's Son
Perfect person to have dust release from Naruto
SI: Laki Olietta
Hashirama. Enough said.
SI: Laxus's brother.
A lot of wizards in Fairy Tail seem to just pump a shit ton of magic into their spells and call it a day. I want to try the opposite of that - low reserves. This forces the wizard to use their brain in their fights and makes them more interesting. Having the OC as a Dreyar allows an excuse for magical talent in learning multiple magics - things like phasing, Nanami's ratio technique from JJK, Yajima's paper body, basic fireball, basic elemental techniques - and also allows for some conflict to form with Ivan. Note that the SI needs multiple diverse techniques or they'll just be weak. You could consider just giving them a broad magic like solid script or chrollo's ability from hunterxhunter as well but I prefer multiple magics.
SI: Erza's brother
If Irene had a son that looked like his father, she'd probably curse him. With her enchanting magic, she can take away his magic and make people hate him. But taking away magic would just result in the kid dying quickly, so she had to essentially enchant his magic to immediately leave his container and hide in his body. Let's say she does a super powerful enchantment here, and makes it essentially unbreakable (some sort of heavenly pact or something) until she removes it. This results in having no usable magic, but his magic is all around his body and in his cells. This means a super strong body with super senses, speed, and strength. The idea from here would be to form the character after Toji Fushiquro from JJK. Give him some sort of pet that can store his weapons, as well as weapons that have inactive buffs (like a sword enchanted to be able to harm dragons. It's just a natural ability of the sword it doesn't actually require any magic). This could be interesting and be pretty strong. As for being enchanted so that others hate him, this could lead him to not being accepted by guilds and being a bounty Hunter. It could lead to an interesting number of tussles with dark and light mages (including Fairy Tail). Also, we can add something so that he's not completely alone all the time. Mages that are ridiculously strong like Makarov can resist the enchantment, and mages that aren't all human can resist it more too. So like dragon slayers, take over mages, and devil slayers should be able to resist the enchantment more.
SI: Romeo
Rainbow Fire is pretty cool. You could really flesh it out more. Red flames provide heat. Orange flames provide concussive force. Yellow flames can augment the body. Green flames are poisonous, but also to user. Blue flames are cold. Indigo flames can hold form but only one object can hold at a time (think Susanoo though). Violet flames are weak but can cause hallucinations - they don't work in the presence of other flames though. All flames combined creates black, which burns everything. When concentrated into a small ball, the incredible heat forces everything around it to expand outwards. The opposite of this technique is white fire, which - when concentrated - can burn through a point in space. Nature will try and fill the hole forcing matter to collapse. Grey is the combination of these techniques, pulling and pushing everything apart.