Explanation

In the first blank, you need an idea that contrasts with “concrete proposal.” Protean matches nicely. For the second blank, you need a word that also represents the idea of not being concrete, as well as the idea of being unlikely to ever come to fruition.

Quixotic matches perfectly. Note that pragmatic is the exact opposite; you have no idea whether the idea is unorthodox.

Protean, quixotic.