I think there are three key factors that keep "end-game" players coming back.
The "Loot Treadmill" that keeps players constantly seeking the best gear (e.g., Diablo3, WoW, etc.)
Some form of fair player vs player competition (e.g., LoL, Hearthstone, Overwatch, etc.)
Completing all Achievements in the game (self-explanatory)
To that end I would say you have some great ideas Drak.
House Regalia: I love the concept of this idea, I think we'll have to wait for more info on houses benefits and lore to see if this would work.
House Honor/Sabotage and Lore Quests: I would be surprised if this already isn't in the works. It's an awesome idea.
Follower Recruitment/Improvement: This is a fun idea and we would need to know more about the "minion" system the devs are planning before determining if this would retain players or not.
Arena; I agree 100% with what you wrote. Competition is great and balancing it is key.
New Ideas: I think for those not interested in PVP there would need to be a PVE alternative to the Arena, something like a Grand Hunt where you go up against increasing difficult mobs and those that make it the highest get tiered rewards based on performance. This could reset at each season and have different themes each season.
Bounty System : Since there will be such an emphasis on PVP it might be interesting to play with a bounty system that could provide rewards in the form of gold, gear, or house honor for players. This would introduce some complexity to character building since you might be asked to pvp a player who has gear/skills that are your weakness.
I'll throw more in as I get them but I like where this is going!