Increasing interest in PvP is as simple as added the ability to select:

Seals, +2 Jewels/Cards/Stones, 15 AC to each member of winning team, 50,000 Cruor to each member of winning team, cosmetic(aura/title/name changing color).

If they added things similar to these it would instantly become popular again without any other adjustments being needed. (Although, those would be nice.)