VMU units, Chao and Battery life.
You know for the era, DC was very advanced.. having a VMU increased your fun, I dont know how many times I wasted my battery, just to get the damn chao to level up before I put him back in the game lol. Least VMUs worked WITHOUT batteries, since it got powered by the DC.
I mean why couldnt systems after that come out with these? I mean Soul caliber was fun watching your chars on it. Or other games gave you details.
GC was close as some games let you use the GBA to get more detailed info or maps. PSX had a PS Pocket or something that was like a VMU.
Wish PS2 would make one or Xbox, hell.. even PSP would be interesting if they made a LCD addon attachment for info.. imagine.. its possable too. Get a dual info screen, kinda like DS.