I don't think anyone who has ever tried to work in BYOND for an extended period of time would disagree with me, the map editor is long overdue for an overhaul.
There are a lot of things that could be done to improve it- but most importantly I think the biggest core features it needs to be less of a pain would be these:
* 1. A different UI for how tiles/turfs/etc are displayed.
The way tiles are displayed in BYOND's map editor is a nightmare. Currently they are treated like files and sorted in collapsing folders using their parent. I don't understand why this is the primary and ONLY way one can view tiles in the map editor.
There should be a way to view and sort tiles differently, something more along the lines that other map editors offer- for example, Tiled.

A display more along the lines of the tileset in that picture should be possible in BYOND. Now, I'm aware that you can sort of get a similar display by abusing Instances, but that comes with a whole other slew of issues and if every turf and obj has different properties in your game (not shared variables), you can't easily do this.
There SHOULD be a way to configure easy-to-use and easy-to-read tilesets and order them how you want for your projects. Even if it's a little extra work to organize the tiles themselves in to workable tilesets, I am sure many developers would gladly do it to give themselves an easier time overcoming the cancer that is this map editor. Hell, you could even keep the "collapsing folders" type deal BYOND currently has and just have it display the tiles differently.
"But I like the way tiles are displayed currently, and I have many tiles in my project that look the same but have different names and/or properties, and many of the tiles in my project are invisible! I would not be able to use a tileset like that effectively in my project!"
Make it optional.
The best solution here isn't to give us a full-on replacement. It's to give us options.
* 2. A different way to maneuver around the map within the editor.
This is a no-brainer. Currently our methods of controlling the map include.. the scroll wheel and some scroll bars, and the mini-map. Unless you have some sort of extra features on your mouse that let you easily scroll sideways, this is just clunky and annoying to work with.
The simple addition of better "camera controls" would make the map editor a million times easier to work with. Currently, there is sort of already something in place- you can use the arrow keys and the numpad to move around but it is extremely slow and it is apparently incapable of registering multiple keypresses so you can't move the map editor display diagonally by holding two arrows at the same time.
Home, PgUp, End and PgDn also "jump" screens but the way this works has always been endlessly confusing to me due to how a keyboard is laid out vs. which buttons do what. Sure, PgDown makes you shoot down a screen and PgUp makes you shoot up a screen. That makes some sort of sense- and after all, the way those keys are laid out on the keyboard relative to eachother means it should make sense. But when you factor in Home and End (which make you shoot a screen to the left and a screen to the right respectively), it absolutely falls apart. I would compare it to playing a Shooter on a console and having half of your camera controls inverted, and on a different thumbstick. It would be better if screen-jumping was just handled by just holding down ALT or something and hitting the arrow keys. (I forget if ALT is already used for something- it probably is, but you get the idea.)
Except that still doesn't make traversing the map easy. Because the arrow keys are in an awkward spot to reach if your main method of putting stuff on the map is the mouse. You should be able to do all of this with WASD. As far as I know WASD does nothing in Dream Maker, and I don't imagine it's incredibly hard to add this functionality.
I still think you can make it even better though. I should be able to traverse the map and place things with one hand. If it's feasible, you should be able to drag the map by holding M3. I know M3 already has a function in BYOND but compared to what I'm suggesting, I think it's a pretty useless one that could be relocated elsewhere.
Navigating the map should not be this annoying.
* 3. Brush types, Flood Fill, More Tools.
I'm sure this has been suggested before, but the mapper should have more functionality. Maybe it's not feasible- I'm not sure why it wouldn't be but if that's the case then why is it not already in the mapper?
There's no flood fill, there's no line tool, there's no oval tool and there's no alternate brush types or selection types at all. The map editor has a total of 3 basic tools to work with. I don't know about you, but as far as I'm concerned that's just not good enough.
(This one is less important than the other 2 in my opinion, but still worth mentioning.)
As much as I'd love to see this happen, I don't see it ever becoming a priority while only one person has the ability to deliver such a project.