Its a bit more of a challenge now with the new rounded edge bases (i.e. the Warmachine ones) as they are bigger than standard GW ones. (30mm rather than 25mm)
I reckon round ones are more popular than square/rectangular.
I think plinths for 40mm bases would be popular, as this covers large 40k models, LotR models and small dioramas etc
Are you thinking of doing these in wood or plastic WF? From the Tyron&Magda auctions, I think they found the plastic ones were more popular