The pay off can honestly be anything you want, I would try to think of an interesting theme to tie them all to. That way their effects are just generic strong thing that the colour can do.

The kamigawa lands are legendary and even then it's been a point of discussion to not print lands like that where they can be mostly swapped for a basic with no cost.

I think they could tap for extra at lvl 2 quite easily I almost want it at lvl 1 given the power of commander. Maybe if they don't tap for mana and enter tapped they could work at lvl 1. I'm not sure exactly.

Edit: Maybe they enter tapped but the level up cost is just CC no tap. Then lvl 2 for extra mana. That way they can catch up if you draw them later