Summary: Add magicka or stamina cost reduction/replenishment as a base bonus for multiple armor sets, or outside of armor sets, to encourage the use of other sets.
Currently, most builds on most classes require some combination similar to Warlock + Seducer for sustain. Folks in chat often call it BIS or the gold standard.
As a guild crafter, I can tell you that people often want something else at first. They theorycraft a neat build that capitalizes on some aspect of the combat system. However, within a few days I am making them seducer... Doesn't matter which archetype or whether they want to be stamina based, magicka appears to be so interwoven into every effective build that I... make... seducer... Tons of it. So tired of Berezan's Mine.
The only exceptions to the general rule appear to be 1v1 burst type builds for PvP and some specific PvE tanking builds.
The only way that I see to encourage gear diversity is to reduce resource starvation globally. Otherwise, magicka and/or stamina sustain gear will always be used by most of the population, which is really a shame.