AFAIK the only way to get multicore support without breaking legacy support would be Asymmetric SMP (ASMP) where the extra core works as a co-processor. From what I've read the programs using it would have to be specifically written for this support.

But what program would really have a use for it? Maybe MPlayer, but there aren't that many programs that would benefit by the extra core(s).

SMP only makes real sense once MorphOS change architecture, along with all the other goodies we don't have. Until then, my vote (that doesn't count anyway) goes to improving the OS itself.