I feel like i'm just saying things other people said but...
The group tool would be pretty cool, I would quite like to press a button on an object and everything that is touching it then attaches itself to it. This would allow you move things as big as whole bases without having to move every single object one at a time. I think you should also be able to save that specific new object that you have made, name it, then put it in a category and it will appear in that category in the editors menu with a custom symbol next to it. This would save a lot of time if you wanted an object that looked cool and then put it in to other maps you make as a kind of signature as well as lots more useful things. Also if you wanted to separate the pieces again, then you could just press the another button and take out what you want.
I also think that weather would be a nice touch and it could really expand on the whole custom game experience after you make your map. I could think of a lot of things you could do with fog, rain and wind, it could really expand on infection. You could have a scale of how strong you want it to be, for example, you could have a scale of 1-100. With this example, when it is set at 1, the wind would only be able to make a golf ball roll very slowly, the rain would be a very light drizzle and the fog would be nothing but a slight blur. But if you set them to 100, the wind would be able to push any flying vehicle trying to go through the wind head on would be flying backwards, the rain could turn into a devasting downpour and the fog would be so bad that you wouldn't even be able to see anything, it could even make the end of your gun fade slightly. If you used them at 100 you would be able to create a killer hurricane, or you could just use them at 50 if you liked the affect it made.
I also think that there should be a drivable boat, preferably one with a turret, or at least allow something like the warthog or the mongoose to float. I would love to be able to create a big team battle at sea.
I would also like to add a new colour changing system. I think a colour pallet for objects would allow for maps to look a lot better. I also think you should be able to completely change the colour of the object instead of just changing the colour of one thin line that goes unnoticed most of the time. I would also like to save some of the colours as presets aswell.
oh great... i've made myself hopeful now...