If you notice a couple small proposals that aren't looking like they are gaining enough traction individually but could fall under one larger header proposal, is there a way to propose a merge for the two proposals to vote on? Maybe vote on whether or not to merge and a name for a merged proposal?
Internally, there is a mechanism to merge proposals, but it is no longer used in actual practice.
The problem is that users have already expressed interest in a specific proposal. Thrusting supporters of one idea into another proposal has never ended well. Someone who is keenly interested in "1960s-era classic Mustangs" might have zero interest in a generic Transportation site — so piling users into proposals they did not explicitly follow loses any sense of whether they would actually use the site if we created it.
We found it better to let users discuss and pick and chose how best to allocate a subject space. The strongest proposal wins. And if someone wants to suggest a different way to approach and allocate a particular subject space, they should submit another proposal for consideration.