Sustaining the ‘fun’? I think this question can only be answered from a subjective point of view. So how would this be more fun for me? I would say that the upcoming ladder system might keep you ingame for a bit longer. Gives you a little goal to play for.
As for current state of the game and in trying to stay with the current concept of close quarter infantry gameplay i would say: maps, options, movement, atmosphere.
Maps need more alternate routes, spread objectives, meaningful sideobjectives and need to be bigger and less cloustrophobic and should reduce the ‘token-of-exposure’ concept i came up with (token of exposure basically means that there are too many points in the map, that when you are standing at that point you are way too exposed to enemy players aka exposure level on some points in the map is simply way too high, even if you know how to properly move) + the objectives should be fundamentally reworked and be made more interactive and interesting and be connected to the theme/story/setting of DB in a way so that it makes it exciting.
Options need to be more, but not spam. Simply stuff to keep you occupied if you are not a frag-type of player. DB very much seems frag-focused. You can decide to play for objectives as an alternative. But what else? In W:ET the huge variety of custom maps played and important role of keeping me interested. Also the objectives are problematic, as explained above in ‘maps’.
Movement. What can i say, i think this has been discussed to death already. Even if the movement system will not be what i want it to be, i at least want slowdowns removed and someone who really opens up W:ET and DB at the same time to tweak all the values neccessary so that the basic character movement feels up-to-par and more fun (, as an example not being able to sprint while moving sidewards feels not good when at the same time you try to promote a tracking gameplay, because with tracking you need to strafe effectively and be able to dodge shots by going shortly under whatever cover you can temporarily find in a fighting situation; another example is that transition of movement direction (left to right for example) is too slow and not responsive enough; again my advice would be to just open W:ET side by side with DB in windowed mode and nail it).
Atmosphere: theme/story/setting, i want some ‘good enough’ reason to fight a conflict.
These things fixed, especially the mapdesign, would probably keep me interested. Would say at first i want bigger maps, then i want non-linear paths and side-routes and lots of interesting objectives. But that would be probably too much of a pub mapdesign. I see that the maps in its current form are especially good for comp, so in that regard the mapdesign might turn out well. But then again, maybe not so much, because there is hardly anything surprising to do, since the paths are just linear and i might imagine if matches will get shoutcasted that this will be a major problem.