The current system allows a proposal when
(# questions in target tag) >= (# questions in source tag)
The proposed new behaviour would allow a proposal when
(# questions in source tag) / (# questions in target tag) <= d
d is decimal constant, probably somewhere in the range
2 <= d <= 3 (open to discussion, certainly).
What this means is that you still wouldn't be able to suggest a synonym that maps a 2,000-question tag to a 1-question tag, but you would be able to suggest a synonym that maps a 2,000-question tag to a 1,000-question tag.
This wouldn't open up the system completely, yet it would allow users more flexibility to put in the "right" synonym immediately (i.e., using a properly hyphenated version tag instead of the one with no hyphens; or using a plural version over singular). Here is a good example where I think the synonyms should be reversed that would be covered by this feature request.