Specifying a specific pair (option2) or even half of a specific pair (option 3) are previously-underappreciated parts of the present system already. Nothing prohibits suggesting a pair of tags, for whatever reason, and whenever a single tag is specified, I already appreciate incongruous combinations with other tags.
Although that's me speaking as a user who wants to make puzzles, not as someone who will have to deal with figuring out exactly what is and isn't "unconventional tag fusion".
I feel like it's pretty obvious what is and isn't "unconventional". Worst case scenario, we have to set a maximum number of puzzles that those two tags are used together in.
