Hmm...that's a pretty cool idea, though I'm not quite sure how you think it should be done...
...if a tower gets destroyed, you have the option of buying a "replacement" that will take the place of the destroyed tower?
...if you have X amount of gold, you can buy a tower and it will appear wherever you select on the map (near your base)?
...if you have X amount of gold, you can buy a tower and it will appear in a predetermined place near your base?
*Predetermined here means that the tower will appear in a certain spot that has already been determined as a "tower slot" for that specific map*
As I said, I'm not quite sure which way you're thinking of having this implemented, but it is a pretty cool idea no matter which way it's done, IMO. I wouldn't mind seeing this implemented into the game, either through a patch or through a Mod. The maps have been feeling kinda boring recently, so having new towers being purchased and erected would definitely help to break the monotony.
+1 for a cool idea, though please elaborate more on it.