one of the things i rather enjoyed early game was the challenges. 

once you run out of challenges and complete the main campaign mission for arbiter, the game starts to feel a little stale. 

if you did a refresh on the challenges every month or so, and maybe a refresh on the campaign in some way to make it feel like we are still working towards something. 
wouldnt really even have to be super awesome rewards. a few appropriate books and a couple crystals after completing a series just as it was before. nothing earth shattering but interesting enough to make it worth the effort. 
maybe challenges could refresh every month or 2. 
i personally just enjoy logging on, seeing a few extra things i can do each day outside of the standard grind, and get em done. the more that are done forever, the less interesting the game becomes and the less likely i am to drop coinage on it. 
arena just doesnt cut it, nor do the boring tournaments that just mean run the same dungeon 300 times in a weekend. 

the challenges offered enough variety that they didnt feel like a grind.