I noticed that most of the Lisa+Nelson fics aren't even in English. As a hardcore shipper of the two, I had to try and change that. Kinda based on a headcanon of mine. First Simpsons fic ^-^
Also I like to think Martin has a little crush on Lisa so yeah.
And the episode Lisa's Date with Density was originally made as like a joke episode but there was one point where the writers and voice actresses for Lisa & Nelson (Yeardley Smith & Nancy Cartwright, respectively) were all just like, "Shit, I ship it."
_
On that one Valentine's Day in elementary school, Lisa Simpson had developed a crush on Nelson and even asked him out. He had always wondered why she went for him when she was clearly out of his league.
Everyone knew that the mere thought of Lisa falling in love with the school bully was preposterous. Some, like Milhouse & Martin, refused to believe this was possible at all as they were jealous of Nelson.
But the part no one knew about she and Nelson was that Lisa didn't fall for him first.
Nelson had started to notice Lisa earlier in the school year. At first he thought she was just the female version of Martin Prince, but then he saw that Lisa was everything he didn't know he wanted. She was pretty, intelligent, talented, and really likable. The complete opposite of Nelson. Neither of the two realized it but their relationship was a lot like that of Lisa's parents. He was just your average stupid, fat, and lazy guy, she was the one sent to straighten him out and deal with him.
On Valentine's Day in 4th grade, Nelson thought this was a perfect time to ask Lisa out. Sure, the guys would probably make fun of him for liking a girl, but who cares? They didn't have to know. But unfortunately Lisa beat Nelson to it by having Milhouse send her Valentine. This resulted in awkwardness between Lisa & Nelson and a trip to the hospital for Milhouse.
_
Later the same year, Lisa had donated money that Grampa Simpson gave her to Nelson. He got rich and tried to make bikes and sell them, but everyone who bought a bike got a refund, leaving Nelson with not that much money left. Despite this, he decided to spend it on Lisa. Together they went to the skating rink. And that night Lisa still felt a little spark like when she and Nelson were still a couple. Maybe she still did like him after all...
There was no denying Nelson liked her still. At first she was the one who wanted the other, but after their break up it went the other way around.
But Lisa had always gotten tons of boyfriends, so Nelson thought that she'd just forget about him completely like she did with the others.
What he didn't know is that Lisa didn't do that. Every time she saw him walk down the hall at school she thought back to their past together, questioning whether she truly ever got over him or not.
_
Things didn't change between them in middle school, high school, or college. Milhouse Van Houten had asked Lisa out for the millionth time in college and she actually said yes that time. Nelson just sat there, silently suffering, as he watched Lisa end up with a dork like Milhouse.
And years after their graduation, Lisa got married to Milhouse. Nelson received an invite to their wedding but just couldn't bring himself to show up. After all, it isn't easy to see the girl you've loved since elementary school get married to someone else.
The marriage was just as stressful for Lisa as it was for Nelson. She knew she didn't truly love Milhouse in that way. But she didn't want to die alone. Plus they had a daughter named Zia together. She couldn't just divorce her husband whose parents divorced themselves and leave their daughter! Lisa really did love Zia with all of her heart. She was her daughter for Pete's sake.
But when little Zia had reached her twenties, Lisa realized that she could finally talk to Milhouse about this without putting too much pressure on their daughter.
Needless to say, Milhouse was a bit heartbroken, but he understood. If Lisa wasn't happy with the marriage, he knew that it just wouldn't be fair to keep her in this rut any longer.
So that night, Lisa & Milhouse had a talk about it with Zia. By now she was a completely independent woman. She was a lot more accepting of this than Lisa thought she'd be. Zia said that it was bound to happen soon, as she had noticed many times throughout her childhood that Lisa was very stressed out. In fact, she had overheard a few conversations Lisa had with Bart saying that she didn't want any of this to affect Zia like Luann & Kirk's divorce affected Milhouse many years back.
Lisa had reunited with Nelson shortly after. She was a lot more relaxed and noticeably happier now that she got rid of all that unnecessary stress. The two had only gotten married 3 years later, as neither of them wanted to rush it. They had a daughter together which they named Luna Marjorie Muntz. Luna's older step-sister ended up getting really attached to her. Neither Lisa, Milhouse, or Nelson thought things would turn out the way they did.
But every single day, Lisa & Nelson were happy to be reunited and have a wonderful young daughter together. Nelson knew it was worth the long wait to get the girl of his dreams.