These are some good ideas, out of the bunch the base building ideas seem like the easiest to design in the current engine. Have you ever tried making mods for DayZ? This would be a good first mod, adding more base building assets. Bikes seem to be a little more challenging, but it depends how you go about making the feature. The cheapest way to do it would be to just toss the whole idea of a bike out the window and make it a dynamic "effect" instead, in the game logic that just makes you move faster, (think: Epinephrine Auto-Injector). I don't know how much lag helicopters and bikes would introduce for players, but I've played servers where there's equivalent assets added to the game, and it's smooth so this isn't unrealistic performance wise.

However that brings us to the last point, "DayZ is a $50 game"... yes this is true, and what you may not realize then having made this point is that you are already getting your money's worth, and then some. I don't know if people genuinely appreciate how much work has gone into the game to fix the major bugs that have existed in the engine. Besides the striving for stability, you also have a team committed to bringing more features into the game. Who are also generally pretty accomidating to the modding community, and help with low level technical debugging from time to time. There are games with larger player bases which expect you to spend more and they would not do this, and they give their players less content.

DayZ has given us so much content, the photo-realistic landscapes, bleeding to death under the most beautiful sunrise, smacking and yanking on your rifle praying you can get it unseized before this guy pushes your corner. There really is a lot of content in the game; if you compare it to other games which have done anything similar you'll begin to understand why it feels like DayZ could be more. I agree with you, but the way we get there is as a community.

Personally, with base building specifically, I've had some ideas of how to majorly improve the mechanics in a "cheap" way (which is important for performance) if you're interested, and would like to see so much new high quality content for DayZ that you can't find anything to criticize, show your support by liking this comment, and let the devs know you would be interested in seeing the DayZ tools work on all systems, not just microsoft windows, whatever that takes.

Summary: Try making the mods yourself, the base building you want is a good first mod. There are "build anywhere mods" you could pull inspiration from, then you just need to make custom assets for the stairs and whatever else you want.