Here are some things that might make things slightly easier
New folder option in dream maker and the ability to change positions of files so there out of abc order.
The ability to open a file in the tree as a dm file even though its not. As map files and interface files are plain text.
The last file that was opened is the default file type of the new option under file. In other words if I'm modifying code and hit new generally I would want a code file. If I'm modifying dmis I would probably want to make a new dmi file.
Another feature that would probably help out is when a map file is unreadable the compiler opens it up as text and points at all the errors so you can see whats going on.
Something I find that bugs me with the interface is when you make a control it makes itself a default size. It probably should be made so you can click the control, drag select an area, and it creates it to that areas size.
I also think the interface needs some off-stage place to work in, a set to window height option, and a set to window width option.
I would also like it if the .dme file listened to my changes. If I want to switch two includes around to remove a used before definition error it would be nice to easily switch them around.
It would be nice if CheckPassport and IsByondMember, didn't require them to be logged on. There are workarounds though.
A new proc that might be useful could be one that remove non-keyboard ASCII from text as a few byond procs cannot handle them.
An option to have the users key randomly assigned to them regardless of their actual key,computer, or any information in a way that lets people multi-client with ease.
The last thing I would like to see is for you to be able to see all the files in a library and modify them as if they were in your source code but in a separate file.