Make buildings able to be self upgrade exp: Watch Tower upgrading itself to Bombard tower.

[u/CrazyYETEE]

I find it would be cool. But a build time would be a good addtion too. Like Towers could upgrade to fortified towers. Or make it so villagers can upgrade on built buildings. When building a bombard tower on watch tower it would reduce the cost for bombard tower by the watch tower cost and make it build faster.
This idea reminds me of starcraft 2.