A new game-mode really seems like a lot of work, with potential balance issues etc, and wouldn't really be something I would expect to happen. More of an idea for KF2, if that ever comes to pass.
A new weapon or two, or even a new specimen seems much more within the bounds of reality, although you still have to consider that these things take a huge amount of time to balance and implement, and don't make Tripwire a cent, so unless they do a U-turn and start charging for this kind of update (bad idea) it's not something I would ever really expect, especially given the age of the game.
The seasonal events are probably much more worth their time, they might still require a lot of effort but they clearly generate a lot of sales (combined with Steam sales/promotions), and the character packs add a bit of icing on the cake so they can earn a few pence from the long-time players too. But still, the game has been out a long time now, you really can't expect them to continue updating it forever, so I'd be perfectly happy if we never got an update again. I've certainly got my money's worth many times over.
There's an SDK of course, so there will always be a source of new content as long as modders are interested in the game. And you never know, if any mods get sufficiently polished and balanced and TWI find the time they could be given the official treatment.