Maybe a PvP Roulette (separate from Frontlines Roulette) would work.
For example, if there are 8 people in queue, unranked Feast will pop (although I prefer the return of 8v8 mode for this... to allow flexibility for choosing jobs in-game, and also promote casual PvP in Feast). If there are 24 or 72 players in queue, Frontlines will pop. If there are 48 players in queue, Astrologos will pop. ^_^
