I have quite a few ideas what more content we can expect. Some of these are firm beliefs, whether others are little hopes based on what the game lacks (to some extent). Also, each new update may contain more than one of these, so it's not like we're expecting 10 more updates or something before BETA. There surely is a lot that can be done (and most likely will be done), but we have to be optymistic about it
1. Balance the current framework. Self explanatory really.
2. Add new items - this includes mods for tools and weapons. The system is there to create a wide variety of things, especially with the new buffs. After initial balancing is finished there will be enough ground work for further enhancements. There could be also added generic items to smoothen the progression or...
3. New foods and drinks - considering the variety of buffs that can be added, this could serve as an extension on the current buff and farming systems. Perhaps new plants to use in variety of new consumables.
4. New vehicles - A17 so far proves that implementing new vehicles will be at the very least plausible, if not way easier than in previous Alphas. As Roland stated, a raft is one thing (especially as it's only on water), but there could be more perhaps some more ground or air vehicles.
5. Perhaps some new weapons - Not too keen anything like that may happen, but considering there is room for adding more weapons we can expect something new. Perhaps new types of blades or blunt weapons. If not new weapons, perhaps some rework on the existing ones.
6. Factions - Considering you can get quests now, extending it towards getting influence and use it somehow to progress won't be too hard to code (i think). If it's in the works, we can expect a more advanced take on this topic, not simply quests.
7. Bandits and survivors - As we are expecting bandits, we surely can expect other survivors as NPCs (perhaps even quests to save them) found throughout the game world. In my opinion an extension to the factions system.
8. New traders - We just got a female trader (though her voice is a bit low), but who knows if we can get more of them?
9. New POIs - From A16 we are getting more and more POIs to play in. Who knows what TFP is cooking in their ovens in that matter. Some of the old ones still need reworking, but perhaps there will be new ones...
10. Smell system PLUS - Getting back to the idea of using smell to sense the player by the zombies. Currently we have sight and sound, which i think only requires some balancing for A17 stable. Getting more ways to be careful in the city may add more fun, as well as more challenge.
11. NEW ENEMIES - AI has been reworked and with some balancing it's plausible there will be new zombies (apart from bandits) or even new animals to hunt or run away from.