Hmm.. I hadn't thought of that, but I suppose it is possible with a side dungeon that appears relatively early.
Worst case, I could always move Tower:1 & 1/2 up to 2 & 1/2 or 3 & 1/2 and move the new branch up there too. Many players struggle to even get to Tower:3-4 reliably, so I think people would be reluctant to ditch a game at that point just to reroll (and possibly get the same dungeon again.)
That said, I don't wanna do that unless/until it's a problem. It'd be an easy move to make, but no need to make it preemptively.