Physical size, so strength in gears etc, high current capabiity, high stall current, low gearing, and above all low impedance.
If you want a substantial performance increase - and you do - you are going to need to break a few eggs. Basically a mobility vehicle has the wrong everything to stand much chace of being useful off road. It doesent much matter which parts you upgrade individually. The whole system needs to be different to see any major advantage.
I would not really consider electric power in any real off road capable device because even if you build something with adequate brushless, high power motors and higher voltage setup etc, and figure out how to do a high voltage brushless controller system - not simple, then its still going to be a little battery limited for REAL off road/steep slopes etc. A petrol powered lighter solution makes way more sense. And 10x cheaper. Theres lotsof stuff available cheap.
Wills 48v BRUSHLESS lithium, low geared, low impedance setup, is a wheelchair, needs to go indoors so batteries make sense. It is basically as good as you will do and headed in the right direction. Thats the sort of setup/thing you must be looking at for any kind of serious off road use. It works because higher voltages and slower hearing increases efficiency enormously. And lithium can then cope.
Have a read of wills thread, and he roboteq brushless stuff in the roboteq thread etc. To get some idea what we are talking about.
viewtopic.php?f=2&t=8845And take a look at his website. He needed to do this because like you he wnted more off road capability. And small incremental improvements are pretty much a waste of effort and $$$ when you look at the system as a whole. Will did this as he lives on a farm, works hard in both engineering and his home. A standard brushed 24V powerchair doesent cut it.