If I was able to pick everyone to always return and everything, my ideal things would be this:
Batman 2: Two-Face, Sal Maroni, and someone small that nobody cares about like uh...The Tallyman or some shit. Also, show Scarecrow getting captured at the beginning.
Batman 3: Joker, little cameo by Lyle Bolton, little cameo by Harley Quinn
Batman 4: Grayson, Riddler, and Mad Hatter. At the end, Grayson takes up the Robin identity. Also introduce Hugo Strange but as someone who works with Hatter and Riddler, or someone at Arkham, idk. He shouldn't be the head of the villains but he should be in there somewhere.
Batman 5 (from here on in has Robin as well): Rupert Thorn, Bane, and Firefly. Thorn as the intelligent head, Bane vs Batman, Firefly vs Robin
Batman 6: Mr. Freeze and Clayface. Make it so Clayface is insanely powerful. At the end, Batman convinces Freeze to help them, so he freezes Hagen and that's how Clayface is taken care of.
Any others after that would include the other ones, but some I probably wouldn't want to see. Catwoman I think they did perfectly in the first one so that's fine for me. Although I think it would be a really cool idea to have a cameo of Selina Kyle.