Sounds like a GAE idea to me (I find the way that sounds somewhat humorous). But seriously, the GAE team has been working on stuff like heroes and branching tech trees.
For a a non-GAE solution, it might be possible to create a resource with which you start with only 1, and cannot produce any more of. That one resource could be a requirement of a titan (a sort ot titan token). Once you build a titan, your token is gone-zo's.