Forgive me for being "Old School" - I began playing M.U.L.E. on my C64 back in the late 70's with a group of friends. This is the ONLY game from those days that some of us STILL play (Emulator).
I think that the main reason that we still enjoy M.U.L.E. is tied up in the competitive plot grab, followed by the turn based development phase that is followed by the interactive auction and resulting ranking.
I like the idea of more players, added race(s), and bigger area. I also really enjoyed a hunting, and catching a Wompus, which gave a player extra money - perhaps this feature could be added and the result of a successful hunt might give the player more time - which would serve the same affect of a Power Up.
As to greater valued land -since it takes longer to get to the outlying plots (especially if the board is enlarged) then perhaps the further away a plot is from the market should raise it's value.
As to time limit, on the old C64 in tournament mode a game took just about 1 hour. That seemed (and seems) to be an excellent target time to maintain. Enlarging the area and/or adding more players while maintaining a turned based development phase may well push game time to 1 and a half to 2 hours. So perhaps a Simultaneous Development Phase? Which could open the game up to races for a limited number of M.U.L.E.s. In order to maintain the current reversing of turns when fewer than 8 M.U.L.E.s are available, perhaps the players could be "frozen" for a limited time, with each player becoming "unfrozen" in the order of Last Place Player first and the First Place Player last?
While we would lose the turned based phase, we would gain another interactive phase. Maybe not such a bad trade-off.
I like the concept of a 3D playing area, not certain of how it would actually work - especially for those with slow connections.