Things in ISP that I wish EP had:
1) The ability to see when enemy turns will take place.
2) The minimap instead of the compass
3) The non-battle system menu.
4) Voice actors who don't sound like they worked in Atlus USA's laundry room and did the entire game's recording while blazed one Sunday afternoon.
Things Persona 2 needs from Persona 1:
THE ABILITY TO TELL DEMONS TO FUCK OFF CUZ I'M SICK OF THEIR SHIT AND DON'T WANT TO TALK WHEN I HAVE TWO MINUTES TILL THE BUILDING GOES UP IN FLAMES.
Things Persona 1 and 2 need from Persona 3 or SMT:
The ability to fuse Personas to get new ones.
The ability to resummon Personas I already had from the compendium.
Ultimately, though, I have to say I really, really hate Persona 2's battle system. Not just because of all the menus, but there's also this way it encourages you to set it up for automatic and no matter how you configure it it feels like this boring slog. Persona 4 (and 3P) have a much better battle system, and heck, once I finally figured out the rhythm of SMT4's battle system it became far better, too.
The thing about Persona 2 for me is that Innocent Sin is riddled with good new ideas and terrible bad ones piled atop Eternal Punishment's sort of dated and lack luster combat anyway. I can see why people don't mind it, because Lord knows I have a fondness for some of the older, clunkier Final Fantasy games, but the game's gameplay definitely has never clicked with me.
However, I am fond of the story. Innocent Sin has this insane, comical, over the top approach to rumors that's perfect for its upbeat, wacky teenage cast, and Eternal Punishment has this more reserved, sort of political/crime drama more fitting of its (still often wacky) cast of grown ups.
I realize this is sort of a slap in the face to the games, but I think these games getting modernized gameplay would really make them much more accessible. For me at least there's nothing about the plots of Persona 1, 2IS and 2EP for them to need to disavow. But the gameplay is an undeniable barrier to many newcomers.