There was a "wishlist" topic, but it seems to be outdated so I'm updating my wishlist here.
I have a some new things I've added to my wishlist and I think at least some of these things most players would like to see.
~ Deleting Maps or Replays (ingame) sends them to Recycle Bin.
~ Byte limit counter in map editor for embedded map items.
~ "Grass Cutter" tool : The ability to mark map areas to keep clear of grass.
~ Option to remove grass from under items (like a checkbox: [

] Item Removes Grass)
~ Ability to create "Gate Items" like Start, Finish, Multilap and CPs with the in game item editor.
~ Ability to rename items from within Item Editor.
~ Ability to "Clone" or "save as" items from within item editor. (to make modified versions)
~ Vector paths in map editor for object items. (PingPong or Looped - with speed and angle settings at nodes similar to custom camera paths)
~ Ability to create custom "Sign Items" that are skinnable like the normal sign blocks.
~ Embeddable sign skins (adverts) with byte limit - (locators become broken over time)
~ Slow Motion, Rewind and Fast Forward controls for replays.
~ Slow Motion option in Mediatracker.
~ Return of Official Stunt Mode.
~ Return of Official Trackmania² Title.

[Edit/Added]
~ Ability to "snapshot" item icons from within Item Editor. (Similar to making Macro Block icons)