General

Need Dbus support for stuff like systemd integration. Waiting on my new dbus library. https://github.com/igo95862/py-sd-bus
Maybe port GUIs to Dbus integration. Less spagthetti, GTK and Qt will be written in native languages. (C and C++)
Some kind of dependency system for services? For example, both wayland and x11 want toolkits settings so they both can depend on toolkit settings service.
Passing file outside of sandbox to be opened by another application or sandbox. For example, being able to download a torrent from browser sandbox and pass it to torrent client sandbox. Use xdg-desktop-portal for this.
Free up temp files after the communication with helper has been established.

Applications

More profiles for commonly used applications. Maybe check Arch Linux stats for commonly used applications?

Services

Use sd-device to search for device attributes. libudev is actually deprecated. sd-device can be added to my D-Bus library.

GTK

How does directory metadata is shared. If you sort a directory by size, it is somehow is remembered when not sandboxed. Is it a Dbus API? a file?
Is sharing recent file metadata folder safe? (under ~/.local/share/recently-used.xbel)
Accessibility bus. ..* Talk to org.a11y.Bus dbus. ..* There is also a GetAddress() method? Is it needed?
Bookmarks. (~/.config/gtk-3.0/bookmarks)

Wayland

Being able to spawn an independent Xwayland server. Needs compositor support. How do I ask GNOME team to consider such option?

Direct Rendering

Being able to select which DRI device to pass. (not sure how I can test without having access to PC with two graphics cards)

GUI

Error message screens
Options window becoming too large as more and more options are being added
Same with profiles drop down

New ideas

Being able to run ephemeral home directory that gets destroyed after closing.
Preload certain files in to memory to speed up startup? (looking at you Steam) vmtouch exists but is not packaged in Arch.
New command terminal. Spawns a terminal inside the instance.