Instead of having a weekly rotation, they should just make it so that they majority of experience points comes from the roulette, with no daily limit on the number of times you can receive it (similar to the adventurer in need bonus, but for everyone).

So, instead of getting say 10% of a level for doing a frontlines, you'd get 4% of a level, +6% if you signed up with the roulette. That way you'd be potentially getting a different frontline map every time, instead of just having the same one all week long. And if people wanted to queue for a specific frontline (like if they're farming achievements for that map), they could still do so without disruption.