I have various things I would love to see in future releases (I think I can't write everything right now, just those what came in my mind):
Wayland support. Wayland is the future, and the present, since Wayland 1.0 released in october 2012. (5 years ago already!)
Better HiDPI support.
Newer software releases backported to Ubuntu, like newer LibreOffice. At least bugfix releases. I don't see any reason, to not update LOo 5.3.1 to 5.3.6 on 17.04 for example.
Some MATE programs are behind in quality compared to its Gnome counterpart, or there is no alternative at all. Nautilus is looks much better and handles icons better too, gnucharmap also looks and works better, Geary doesn't have alternative at all, etc.
Maybe a bit too futuristic, but MATE mobile is something I would love to see.
Currently there is only Plasma mobile, but IMHO that looks horrible. I don't like KDE's design, but that's even uglier than regular KDE stuffs.
Integrate menus, window buttons and the top bar into one single bar, like on Unity.
An audio input/output panel, where I could assign every app separately to every output, and I could use two outputs parallel. Like play music on HDMI, while playing with a game using the notebooks internal speakers.
An improved Plank (I detailed it on a separate discussion)
An improved alt+tab panel. Currently it has only tiny icons, and nothing more.
When I minimize a window, there is a weird frame collapsing animation.
Improved multi monitor support.
For example the ability to set default launch display for every program. fixed to a specific display, or the last display where it was used or the display where the mouse were when the program were launched.
An improved advanced menu. The current looks and feels outdated, compared to, for example, Unity's dashboard. A Synapse equivalent search panel integrated into this menu could be also useful.
On the top menu the indicators and icons are full height, so they fill the vertical space of the panel itself where they are. I really really dislike this. Icons should have use around 75-80% of the available space at maximum.
When I click to the clock, then the calendar appears, when I click to a different place, it doesn't disappears.
Ambient MATE Dark theme looks a bit bad with LibreOffice. for example the UI is full with light grey outlines with dark grey background. Doesn't looks that good.