Oooh, ive played alot of wc3 TD in my days..
Just some gripes:
The grid system really helped out in the placement of towers, i fear blocking will be quite common :/
Use flying builders. For great justice.
Make really flashy fx for the bigger towers, when you see one of those babies go off you know someones gonna get red on them. Take care on the polycount a lot there though, theres bound to be a lot of the biggest towers in the endgame which in wc3 often leads to lagfests.
make the towers interesting. Chain lightning, slow (this is probably the most important attribute), monkeylord style beam, use the dgun button on the bigger towers.
A spellbringer is a nice twist too: A building with LUA'd mana that can cast global spells, (ala the orbital bombardment thingah by imbaczek) which can help you clear your creeps (or the enemys towers

).
EDIT: yeah the creeps themselves are really to be done last if this will develop anything like a wc3 TD