1. Horizontal Gear Progression is a bad idea because it'll lead to gear bloat and having to carry a set of gear for every encounter or be kicked from the party.
2. Adding Sandbox elements can work but that mostly involves having a stronger PvP system, a variety of PvP content and maps.
3. Charater Progression can work if you buff the potency of abilities or change their affects.
4. Adding a variety of progression PvP progression like sieges and PvE progression like Assualts in instance map where we take over an enemy stronghold for relic progression. Garrisons an instance map where we can gain PvP rank.