What if kill gold had Irelia's passive?
Meaning: if you are 1v2 and kill both enemies, you get the full gold (or maybe even a little more). If instead you are 5v2 and kill both enemies, you get way less gold.
This way outplays are still rewarded, but 5 man ganks have less reasons to happen (they still lead to a secure kill+tower, it just is worth less to leave the rest of the map undefended).