Sean is based off of Dan in my opinion. No, he's not supposed to be exactly like Dan but I think they took the concept and make a character from it that's not supposed to suck as hard. I don't have any proof of this, this is just my opinion.
Here's how I think they're similar. His uppercut may not be the same as Dan's but it serves the same function. The 2nd hit fits his basketball motif. He's a shoto with no fireball (Hadou-burst doesn't count) instead of a shoto with a worthless fireball. Dan may not have Sean's tackle but having Sean say "mommy" when he whiffs it is very Dan-like.
But it's pretty clear that he's supposed to be a comical character.
And there's not a lot of chars who were nerfed from top 3 to bottom 3, if there is, inform me.
Alright, but this is just off the top of my head.
Guy from Alpha 1 to Alpha 2.
A few characters like Rose went from top-tier in A2 to not really playable in A3. I don't really know who's in the bottom tier in the game (other than, like, Birdie).
Aoi from VF4:R to VF5.
Jin from Tekken 4 to 5. But Tekken 4 shouldn't count for anything.
Talim and Raphael from SC2 to SC2 patch. Raph counts for SC3 too, I guess.
ABA from Slash to Accent Core. Though that's not quite bottom 3 to top 3. Also that's going the other direction. Anji goes the proper way, but he wasn't quite top 3 in Slash. Close though.
I'll just end there, I don't really want to think about it much more.
But anyway, this was fun yet beside the point. Rachel is a mostly original design while Sean is mostly not. That's more why I think comparing the two is silly.