My (newbie
) assessment:
I agree with all of your plusses
"1- you need to build/aquire a differential. If you aquire it and it fails, diffiult/impossible to replace/repair."
I would agree with you, there.
"2- the three wheel design may be prone to tipping, especially when on the ramp."
Add two caster wheels on the two previously "un-wheeled" corners!
"3- an omni wheel, as monsuier(however thats spelled) coffee calls it, will need to be machined. . . . we would probably use rollerblade wheels or something, tho, it would need to be able to get across those little 'lips' on the start of the ramp easily."
If your omni is behind your main drive wheels, it will have more traction than it would on a level surface. Roller blade wheels would potentially grip relatively well if you made little nicks in them, I would think...
Overall, it sounds like a cool idea