I think it’s a toss up between Mac&Linux port and the Co-op multiplayer.
Each of those mean a huge reworking of the engine (unless they already built it to support it, in this case, ignore this) and that is the last thing you want to do late into a project. Yes, it’s still pre-alpha, but even then…
Then maybe planes, depending on how they decide to realise them.
All other goals are just adding content and assets, which is (while time consuming) a straightforward thing to do.