Quick note on Hostage since this turns up all the time in piecemaking: if the idea is really good for ally attack, you can just pretend hostage doesn't exist, since it is technically possible to remake hostage to conform to that new unit by having some enemy-only behavior, rather than the suggestion needing to conform onto hostage and potentially never existing in the first place because of it.

If you are aware of hostage and still want to make a suggested ally-kill type effect, you could just indicate somewhere '(except hostage)' and everyone will know you at least realized the problem in its current form.