I'd like if they finally released Duke Devlin, the Orichalchos guys, the big 5, PaniK, Strings, Pharaoh Atem.
They just released Rex Goodwin for 5ds, so I understand why they wouldn't want another so soon.
But if it had to be a gx character I'd like the other overseas champions, Chumley, Thelonius Viper, or the Shadow Riders.
Probably not the Supreme King, since we got 2 jadens right now, but eventually since I want the Evil HEROES.
They're probably not gonna release the orichalcos guys or the big five, mostly because the orichalcos guys used cards that don't exist, and because the big five's cards are pretty much all already in the game, so there would be no point. But also, these are all filler villains, so even tho some of them might be cool, they didn't really do much and they didn't matter much outside of their arc.
NONE of the villains were relevant outside their main season, though. Pegasus and Marik played minor roles in later seasons, but I wouldn't consider their appearances noteworthy. Bakura showed up a lot before he was the main villain, but really did little of actual importance until the last season.