As a Mentor I should be allowed to swap my role to another role should the party need it. Too many times you're doing content and a Tank leaves and I'm thinking, I could tank this if the game would just let me swap my role. Obviously you should only be allowed to swap to a role that is missing so people cant swap when it's not needed. This would prevent a lot of parties from abandoning. The only other way to get around this is for me to leave, swap to Tank and sign up again which leads to my second point....
Mentor roulette should prioritise backfilling content and not starting new instances. A lot of times when I leave an instance, swap to Tank and sign up again, it will try and put me into a fresh instance which will either cost me 1 of my 3 abandons a day to stop, or for me to accept this new queue and leave a party that needs a Tank stranded forcing them to vote abandon.