Here's my honest opinion on these suggestions so far. Some I like, some ideas I've already had, and others need thought out more.
1. For the first one, I've stated already it could be useful, but my concern would be that the tile system would become overcomplicated. It would need done in a way that makes it optional to not add overhead to games that do not use it.
2. Both of those we've wanted for a very long time, except the texture editor. I already added the ability to load a texture into ENIGMA without also treating it as a background or sprite.
3. That's a good idea, but I wouldn't put it in LGM. That was similar to the goal of the Synth look and feel, to make a skinnable look and feel that can be styled with XML. A designer to make look and feels for Swing would be nice, but it would be a better idea to make it its own separate program so other Java developers could use it for other programs. Also, Google the "Web look and feel for java" because it has CSS styling support. It has a number of issues in the current LGM that will be patched in the next release, but it would be best to wait for v1.29 of Web L&F also because it fixes things like resizing MDI frames.
4. Thought of it already, there's also an open feature request for it on LGM's tracker by me.https://github.com/IsmAvatar/LateralGM/issues/110
5. This idea needs fleshed out, I think that's a little overkill. However, I do think the GM manual is terrible at describing data structures. It could definitely use some more thorough documentation on the subject, and so could our wiki. I think it would be helpful to add better examples of when to use the structures and how to think about data structures because usually they each have their own purpose. Another possibility is that it could be use for editing constants if you wanted to make a constant array.
6. I've already had the idea of path branching and it's something wanted, but it would probably differ too much from GM. The idea would be to make it an undirected graph where a point can be connected to more than two other points.
7. Yes, but maybe not as full blown as you mean. GMS did add asynchronous events to handle parts of the multiplayer, but the implementation is pretty atrocious. It would be nice if you could simply drag an action and have the location of an object synchronized with the other players for you.
8. There's already a drag and drop action for that, and it's possible to make a scrolling background in the room editor. I really don't know what else could be added.