I was thinking of a little adaptor like the lego ones
3mm D-Shaft in one end, Hex on the other.
Or, maybe something like the 4mm hub that comes with the Tamiya wheels, but with a keyed shaft hole rather than the round hole they have.
Thought it might be good to be able to easily use any of the Tamiya wheels with the micro-metal and mini-metal gearmotors. No particular wheel in mind.
I guess if it was to support bigger shafts/motors then it would need to be really beefy and probably not worth the cost.