Well, title says it. The only things that you can possess in this game right now are things you do need, armor, weapons. So when you finally got your dream equipment, there's just no reason to buy any more. Which means that at that point, all you need to continue is enough CE to play as long as you want to (and honestly, at that point, most people won't have a problem to buy the CE they need with the crowns they make, at least mostly). Your purse is filling up with crowns that you are probably never going to need again. So, what then? Well, right now, nothing.. except run Vanaduke again.. and again.. and again.
To keep people interested in this game, they have to be able to have other goals than just get through tier 3. They have to have things they can work for, a reason to do clockwork runs.
So I think that what needs to be added are more aspects to the game. Things to buy, things to do that cost you something. Competitions, not necessarily PvP, maybe something along the lines of Legolas and Gimli in Helms Deep - I killed more of those baddies than you! Other things to buy - guild hall furniture, hell, why not your very own knight hall? Your own snipe to keep you company in Haven / maybe even in the clockworks (Yes, of course all those monsters are going to turn to our side when they hear the snipe chirp)? Just things that you certainly don't need to be playing this game - but that do add to the fun of it, because you're going to want them all the same!
The important thing here is that while adding something to keep playing this game for, you create a sink for crowns / CE. Where all those endgame-players can dump their riches while getting something they want. The rate of crowns should probably be higher than that of CE, so that people who want these things may think about turning their CE into crowns.
Because at the moment, I can't see many reasons to do so. Once past a certain point, all you need is CE, not crowns. What crowns you need you get just by playing the game. This ruins something of the F2P-idea - People do need to exchange CE for crowns so that free players may continue to play.
I most want portraits of my knight, so I can show off items that I own (and think look neat, like the slime slasher) without using them in the dungeons.
I also want to see costumes. The Rose Regalia was a nice set and a cool promotion, and now I want more! Maybe simple stuff, like other colors of the chromalisk armor and I'm sure the artists can come up with other neat costumes.
Guild hall furniture would be nice, too. I don't think I'd bother with a personal home, but others probably would, so that's another possibility for people to have some fun.