I'm sure this has been suggested somewhere before. Having towers destroyed and being unable to replace them is a source of constant annoyance to me. I would like to be able to have the option to pay gold to replace permanent towers or to create additional towers to bolster our defenses. However, I don't think it's a good idea to allow players to place towers anywhere they choose.
Each map should have a tower spawn point at each default tower location. If the tower is destroyed, a player has the option to buy a tower from the item shop and place the tower in the tower spawn point. The actual gold cost of a tower should be less than 1k gold, will have to test out the balance of this a bit, but I think anything above 1k is just too costly and might as well get some gear. Spawn points will not be available until the original towers has been destroyed, and not before the proper war rank has been achieved (as some towers spawn at certain war ranks). For example, if one of the side towers goes down early in Cataract, you can replace it so that you can use its strategic purpose and not suffer too much from an early loss of a tower.
There can be tower spawn points in addition to the default tower locations that players can place in order to boost defenses. These will have no towers at their locations unless a player decides to place a tower there. An example would be to place an additional lane tower in Cataract, bringing the front towers to 2, making advance up that lane more difficult for the enemy.
This idea can be expanded to not only towers, but to other unique buildings that provide a strategic advantage, like trebuchets (non-fort version, like on Brothers), or other builds that might not exist yet, such as a permanent version of Totem of Revelation with longer sight range, or permanent (insert stat here) ward increasing allied (insert stat here) in its radius, or wherever your imagination takes you.
I feel this would expand the horizons of the game, increase depth of strategy and tactics, and ultimately make the game more interesting and diverse for everyone. (You like how I just said practically the same thing 3 times differently? ) On the other hand, if it's possible, maybe this can be modded into the game.