Personally I wouldn't be happy if they expanded regular dungeons I only have so much time during the week to run a roulette so I can tome cap.

However I would really like if they changed raids and 24 man dungeons to have a more exciting and open format.

This would allow my self and others to run these dungeons when we have time, perhaps even giving them a roulette, maybe hide a few tomes here and there in random chests off the main path to keep people exploring or a bonus for a full clear.

Making longer dungeon will just lead to people dropping out of the party when a long dungeon comes up in roulette, you see it all the time with ex roulette when you have an imbalance in dungeon length.