I believe a way to keep the game going would be to have randomized dungeons. Minecraft tried to do this, and in my opinion, failed. The mine shafts are predictable, the strongholds are too. If the developers could get the game to, lets say, the level of Skyrim, with randomization, would be incredible. It would also be nice if they had randomized loot and such.
Something sweet would be a way of missions that isn't MMO-ish, if you know what I mean (like "Kill 5 orcs" ect.) If they would be progressive missions that had multiple steps and you had to think them out, that would keep the gameplay fresh. It would be cool if you added in-game campaigns and co-op ones too.
If the game developers decide to go down the path of skill trees, or heavily customized gameplay, they should keep it balanced. If they make one way or another better, then it will turn into "Oh my guns better than yours go make it." I think customization and skills are awesome, but keep it working.
Give me some feedback!