Technically, a DS card of 128 MB (or even 64 MB) would be more than enough to fiit 4, 5 and 6. But, yer right it wouldnt be economically feasible.
Particularly if they feature enhanced versions, depending on how much new stuff they added, they could sell them standalone. Hell, i know I would buy it. -__-
That said, I STILL cant beleive you dont rank Chrono Trigger as classic alongside FF 4 and FF 6. It played better and more dynamically, had great memorable characters, a good story, excellent *excellent* graphics and music composition....dude, when it is said that Chrono Trigger was put together by a dream team of RPG moguls (which it was, featuring the heads of Final Fantasy, Dragon Quest, and Akira Toriyama of Dragonball fame), it is said with 100% seriousness.