Thank you all for all the valuable input! I think I'll move forward with the fork in the second half of February (I'm a bit short on time for now). I'll notify Wikitech-l once I have a repo and a list of tasks set up. :)
As for Vue – yes, I'll definitely keep an eye on that. For now I'm postponing major UI work, as Vue + ES6 (that I heard is coming to RL, wheee) could help with that a lot.