Hey! I put my 4.261 pence in already!
Seriously I wish you the best of luck with getting the people together and "keeping" them together long enough to actually make something either way. I'm sure you will find that the majority will bail after they find out that they actually have to do some work besides impress people with their new titles.
Try starting with a story line and working it down from there to the nitty gritty, if a custom verion of UO will do everything you need to get what your looking for then just do a new version of UO. If not then make your own game engine. Personally I would do the UO EMU first to get your feet wet to see if its what you think it was then go from there after you get the experience.
Dev Viperrious