Sounds interesting. I might give this a go (when I have access to my computer once again...)
But it would be better with the 1.12.2 Customised world type, right? I mean, you could be able to choose « Mesa » as the only biome in the world you would generate. (Or you could also use 1.13’s buffet world type, but rivers won’t generate compared to the Customised world type, so the only wood source available is from mineshafts.)
That being said, once you have a tree and saplings from a river, it gets a lot easier to get wood. Once you have a fishing rod (and potentially carrots/potatoes from zombies or chickens from chicken jockeys), you have a very stable food source. And it goes on...
But let’s say: I want to go for every single advancement possible in the game, or gather every resource possible, which I can’t gather in the Mesa of course. In that case, is travelling to other biomes OK, or is it still against the rules?
(Sorry about the very long post BTW)
PS: please avoid posting spoilers in the replies. It ruins all the fun to me.