You don't need to wait for RQ:Adventures in Glorantha to get some first-rate Third Age material. Moon Design Publications has its excellent Sartar/Sartar Companion and Pavis books already available, and is releasing 'The Guide to Glorantha' this year. The Guid is systemless, so you can use whatever rules you like, and although Sartar and Pavis are both written for HeroQuest, the system stuff is so light that it can be ignored.