Many of the suggestions that the developer recieves are either duplicates or thigns that just won't happen. They may not happen due to a number of reasons, most of which are either A: complexity of putting it into the game or B: available resource abilities.
This is a short notice of things that have been confirmed will not happen for the forseeable furture. The list will be modified accordingly if things change.

Map Maker
Imported Maps
Imported avatars
Imported Guns/Weapons
Gun/Weapon editor/maker
Full water physics
Mods
Editor control changes
AI or single player bots

The developer has confirmed that there will be no custom maps for the forseeable furture for one specific reason: complexity. What is involved in making a map maker is a LOT and that's not something that he wants to tackle right now. Maps can't be made in UE4 and imported due to the garage and how it works. You can't place a garage entity without having a "proper map maker", whic won't happen.

A gun/weapon editor/maker will not happen due to the animations and models themselves. There's no way (currently) to combine the bricks into a single model from in UE4. The files have to be exported to a modeling software and combined that way. Animations cannot be made in BR, either, and have to be made in UE4 or a modeling software and then manually put into the game...which means mods...which means it can't be done.

Changing the controls in the editor just doesn't make sense at the moment. They have not changed since the beginning of the game, why change them now?

Full water physics is just too laggy. Yes, it's been tested. That's not to say that something isn't coming eventually. Last we heard, the developer was toying with a "bouyancy brick" that would interact with the water plane instead of all the individual bricks. This is unlikely to come out soon, but it is (sort of) in the works.

Anything that requires mods (Maps, weapons, skins, bricks, physics, etc.) will not be possible in this game due to the game engine used. According to the developer, UE4 is not mod friendly due to how it manages it's files and content.
(Edit:)
I now know why Unreal Engine 4 doesn't like mods.
Think of it this way:
When creating an asset in the UE4 editor, you are working with raw, or "source" data. This Source data tells the full game what to put where and how to use the asset. Things such as links between entities, animation routes, animation brushes, light entities, etc, all have visible icons, node paths, and similar things that you don't see in the game. That means that with every change/mod you make, you have to recompile the game and "cook" everything together, which removes unessesary and laggy things such as the icons and path nodes and hitbox markers. Unless you have access to the developer version of the game as well as all the assets and the loader, then you can't make these changes. This goes for ANY mods, not just fancy ones. Maps, player model skins, custom images, etc, all need to be "compiled" together and then loaded into the game.
Dovetail Games has been building an editor for Train Sim World that runs in-game, but they have many people working on it and it's taken them several months.
Fluppi is one person.
Even if he did decide to make a map editor or a mod loader, it would take him a VERY long time to complete it.

AI is VERY complex and just isn't viable for a single developer to do. The zombbies are extremely simple and just go straight to the player. Creating a computer-controlled vehicle would take a long time and is very complex, even if it's just following a line on the gorund. Sorry, it's just not going to happen with only one developer.

That's all for now.