I've already said this somewhere but I'm going to repeat myself here.
There IS a CPS1 multi and it's absolutely awesome, made by a french guy who goes by as "aje_fr":
http://www.neo-arcadia.com/forum/viewtopic.php?f=14&t=45917&sid=7b15daf3e1592491c4ec45066e8f6c57
The problem is that the guys seemed to have stopped producing them and won't ship outside of France.
Imho, there's no need to reinvent the wheel, I'd be really really happy to see DS get in touch with aje_fr and distribute his work because, as it is, it's perfect.
Load roms on a SD.
LCD screen to select and launch the game.
*instant loading times*
Uses 1:1 MAME roms, without needing any kind of hack
What can you possibly ask for more?
As for the other platforms, I don't really care that much.
A SEGA System16 multi would be nice, but I have all the games burned on EPROMs and whenever I feel like changing game, I just swap the chips and a couple jumpers on the board.
A CPS1 multi is all I need in my life right now.