Me101, reading your suggestion gave me an idea.
There are three rare building draft levels:
(1) can build level 1-2
(2) can build level 3-4
(3) can build level 5
I suggest allowing usage of the higher level drafts to build lower level buildings such that
(1) can build level 1-2
(2) can build level 1-4
(3) can build level 1-5
You still would have to find 5 drafts to completely upgrade a rare building. And this gives more value to high level drafts.