I like the idea of Alliance Raid Roulette rotating, similar to PVP, where one day it would be the level 50 raid, one day would be level 60, one day level 70, etc. However, I would worry about not enough people queuing when it's the level 70 raids.

The other option I think may be worth looking into is for the rewards to scale, similar to Main Scenario Roulette, where you get more XP for Prae since it's longer. People who are doing Alliance Raid Roulette for XP may actually stay when Orbonne pops if they know it's going to get way more XP than doing LotA or ST.