How about a mode of play where the player can not only choose what props to add to the map but edit the roads?
(I understand this is almost like the_viking's suggestion, I changed a few things however)
The player will be able to choose where the road leads.
What theme the map will be. (desert, jungle, arctic etc.)
What type of enemies will spawn. (A randomiser can be added here like in the ruins of naz'de in frontiers)
What props are present.
What towers and upgrade levels are available.
Various side quests.
Regarding where the roads can be placed, the player will be able to place both the starting line and finish (multiple of both is an option) anywhere on the map and then of course create a route connecting them both as well as choosing where the player is able to build towers.
As for the props, I was thinking about being able to place, as an example, a herde (or single) sheep anywhere on the map except for the roads. A few other examples: dead bodies, dead animals, other wildlife that are alive like cows and chickens, farmers and other characters that don't affect the battle in any way but add some life colour to the map (like the slaves in the ruins of naz'de in frontiers... Okay, that maybe isn't the best example... )
I think infantry towers like the sylvan elf hall and the one with the jungle women with the spears would be good to add to the editor as well as towers such as the shrine of regnos or the sunray tower. Boss characters that taunt the player would also be a good idea too and various secret props that activate when you click on them like the dancing frogd in sape oasis.
(so basically i want everything i love about the game in this editor hahahaha!)
I mentioned side quests and what I mean by that is having the option to add an item like say, a princess and placing her somewhere on the road or in her own special tower, that must be protected like the end of the road, the enemy will try to kidnap her and if they return to base with her then it's game over. Now whether it is a princess, prince, king or queen doesn't matter, could even be a treasure chest, either way it will add a bit more suspense to the game no? An extra challenge! Of course that is just one example, im sure you guys can think of others?
(if anyone thinks this is any good id like to hear your ideas on what props and side quests could be added)