I am currently interested in the Classic 68k line right now. But I know it has very little future viability. No matter how I love it, hardware is getting flakier each day that passes by, and not getting any faster to meet most future or even actual computing tasks.
In the immediate future I think that the fpgaarcade will give a lot to talk and play about.
I believe that MOS and OS4 will continue progressing until they hit the wall with the PPC desktop architecture, and are doomed to extinction at that point, unless they entirely redesign themselves, which given the resources they have, seems unlikely to happen.
In a much more distant future, I see AROS as the only viable alternative, as all others will probably remain as part of an exercise in software/hardware archeology, for not being able to keep up with the quick pace of the current technlogical progress, and become each day more outdated and unbearable to newcomers (Oldfarts like us, that bare with those inconviniences, will eventually die).
I voted AROS, as it is the one I believe has chances to prevail in the long run due to the fact that it is open and portable, which means it is not tied to bad business desitions or any specific hardware or vendor, which to my view makes it particularly interesting.