You have some really great ideas here, D3ad! I really enjoyed reading these and imagining them for myself. Sadly, as you've been informed, we do not have the source code to be able to do most of these things. However, it is important that we do think of things that could be added if we ever do obtain that code, alongside and most importantly what we can add now.
With the mesh idea having them less flat, it would be possible, however it would be very difficult to implement for every mesh and item. Taking for example how trees are made, as they are hollow inside but wrap around into a cylinder. With the help of a good texture, it looks like a decent 3D model. If the same was done with everything, I'm sure it would have a better 3D look to it. I haven't modeled much, if not at all, but going off of other meshes that I've seen, I know it can be done if someone was truly willing to put the work into it.
With the fur idea, that would be really neat to have. It can be done with a custom made item pack, but having it on the model proper, things would have to change. Fur would have to be created and rigged to the model itself, from what I know. The current animations would likely have to be redone, plus new animations for the fur itself. It's possible having to redo the current animations could be bypassed if the fur was created with care, though. I'm not entirely sure if we would need the source code to be able to do this, but, regardless it would be a big project. Once again, it would be really cool to have this if we had the proper resources.
I personally would love to see the old maps return, however, we have to take some things into consideration. Many of the old maps were huge, such as Fluorite, South Pole, Atlantis, and Last Cave. Some of those maps put a huge strain on the server, thus it would crash a lot, and cause many problems. Thank goodness we had Raz to fix them all lmao. The maps were considerably barren due to their size, and lacked the beautiful detail our new maps have today. Granted, the old look of all those maps are what made them so special, they still held many issues. Without those maps, we would not have gotten to where we are now. However, what we can do in the future, is incorporate small things from those maps here and there into new ones. Taking for example the big Z hill, L Island, or any other area you love, in a future map we could recreate those areas within a new biome/map. The same could be done with that cave atop the hill near Stone Bridge, or just taking that whole area itself and locating it into a new map. Many things can be done about bringing the old maps back. All it takes is planning and attention to detail. All in all, regardless of what we want, we need to decide what's best for the game, and personally I think doing those things that I mentioned could help. We'll see what other's think though~
For picking other players up, if we had the source code, I'm sure it could be done with a lot of hard work, and if enough players wanted it in the game. Weather we'll be able to implement it in the future, I'm not sure. It's a great idea to ponder though.
As for everything else you've mentioned, everyone has said pretty much what I would say. Thank you so much for sharing these ideas! They are very good. Do share more if you come up with any!