When you create a template, after naming it, you could put tags that represent the type of construction and could later be used as obetivos missions.
Example: You create a tower defense with the tags “tower” and “defense” so a campaign could ask you build him this tower and the finished, complete the mission.
This method could even be used to filter pre-made templates by Radiant team and other players.
Though I like your idea, it’s kind of exploitable unless you add some way of defining minimum requirements for each structure, which might get too limiting.