I'll pop in to add Runequest 6 is being worked on (by Lawrence Whitaker and others who were involved with Mongoose RQ) and is apparently fairly close to release.
On compatibility, the ones from the Mongoose RQ2 line are:
Monster Coliseum; adding a lot of monsters beyond the ones in the core rules.
Empires; with rules for running everything from manors up to globe-spanning empires.
Necromantic Arts; good, but rather specialised.
Arms and Equipment guide; not as awful as some people say.
Of the campaign material, Vikings is excellent. Wraith Recon and Age of treason didn't really appeal to me - that's down to the type of setting rather than book quality, though. The Glorantha books are good, certainly.
