A few years back I was trying to persuade a pal to start a company with me - making powered wheels. This was to be for cars.
There is only one way for the alternative fuel thing to work, and that's to get rid of all complexity. No I. No drive train from conventional motor(s).
The wheel was to be the inappropriately named Stator. No power had to be conveyed to a rotating part. Two, or even four computer-controlled wheels will be the future...though I probably won't be around to see it.
About the only way I could see a sizable aircraft - and that's most of them these days - being self propelled, would be for several of the main wheels being powered. The idea that the squitty nose wheels could do the job leaves me totally non-plussed!
The main-wheel powered hubs wouldn't be the problem, it's the powering of such devices that will be difficult. Hundreds of cycles of gear-hauling, with zero danger of frayed hi-power cables. Light flexible conductors, that offer very little resistance even after they're loaded. Harder to do than you would think.
Even hotter things being stowed in the wheelbays? Breaks get hot during long taxi-runs, so that shouldn't be too problematic, but as for putting a HOT motor into the electrics-busy area up front. I don't like that at all. I defy a motor that's pulled a 747 a mile, to be anything but hot.
Right now I would put my money on a cockpit controlled removable mini-tug. But when that's been fueled by whatever...and the aircraft has been cooled and aired...the net gain is starting to slip away.