I love the idea of an incubation period.
I like to propose an idea that mimics the process of having to gather a number of signatures before a proposal can be entered.
During the period a proposal is locked, a certain number of neurons must vote to unlock it. Neurons can not vote to keep it locked, but only vote to unlock it.
If a locked proposal does not receive sufficient signatures, ie. unlock votes, the proposal never gets unlocked and is deleted.