This is probably impossible to pull off but...

Bring out new content every month instead of every 3-4 months.
People need new content to keep busy.
For example: Instead of bringing out 5 Coil turns every 3-4 months, why not 1 turn every month?

Bring out group leveling content for both DoW, DOM and DoH, DoL.
The Leve quest idea is good, but the execution is far from perfect.

Stop bringing out dungeons that drop 55-65 gear, nobody actually needs that and people tend to not run certain content because there is no point in doing it.

Arena PVP is the most boring thing ever, bring out 24/24 battlegrounds and try to bring out the Frontlines patch within a decent amount of time.