If you want to keep everything in a single projection, then Mercator is far and away the least bad option. That's why it's used for things like Google Maps and OpenStreetMap. It's reasonable at both large and very small scales. It isn't that good in between (Continents, large countries like Canada), but it's no worse than any alternative choice in that regard.
My personal preference would be to design the world first (Geology, climate, ecology, history, linguistics, etc), and then when it's designed, present it as maps. Done this way we'd be able to use appropriate projections for each map, and where they overlap, they would be drawing from the same source data and so should be consistent. So instead of stating with the smallest scale, designing everything, then subdividing doling out the divisions, and letting each mapper add/refine the details of everything within their segment, It would be starting with the most basic aspects like tectonics, and we'd get that nailed down at all scales then figure out climate, then ecology, and so on. Then when all the layers of information are done, we can make maps. Of course I admit a lot of people would find this approach less fun as the 'making pretty maps' part is delayed until the very end, and not everyone is interested in all the different things that would need to be dealt with.