Some potential ideas of varying complexity:
Keyboard shortcuts should appear if you press ? (SHIFT + /) like Gmail or (CTRL + /) like Slack depending on whatever you prefer. I think this is a easy way to remind yourself, without having to click Menu -> Help -> Keyboard Shortcuts.
Stylistically could add transparency across all the menus background colors, somewhere between 60-70% would do. Feels less like you are leaving the game every time you open a menu as you can still see underneath. A couple times, in chat, especially I have wanted to be able to see what someone is talking about in the game, but I have to keep opening & closing chat, especially on mobile / tablets.
Theme colors for menus that you can pick would be nice as well (someday in the future) I'd love to have a Dark Black or Red theme instead of the Blue.
For the single player missions on the main menu you should change the circle skull icon based upon the level of completion (different symbols for normal, hard, nightmare) so that you can identify what you've completed. Probably should also display your best completion rate for each level when you click into the map so you know where you need to improve.
Adding a preview image to each of the maps so that you don't have to load into a game each time both for multiplayer and Single player maps. Lets you get a feel if you want to play that scenario.
Profile pages for users. Allow you to click into them from high scores. This would help build community, you can see the information when you are in game, but it would be cool to see more about who theses high scorers are.
Game History / Replay Systems these are long term projects enough said for now.
2x (or Nx) speed in multiplayer games. This could be limited to an Admin function in custom games. This would be ideal for if a group of folks were getting together to play.
Maybe a search function / overview page of the immortals that exist on a map. Very useful if you need to know how many Immortal Elves are on the field for the Undead Ents to feed on. I was thinking search might be nice so you can find the hydras and dragons really quickly!