With the introduction of Seasons, PVP is being integrated into the game into a more interesting manner. I would love to see combat tied more strongly to the strength of a person's harem. Currently, the ways to become stronger are fairly limited. You can max out the 3 girls you use in combat, buy stats in the market, find a good club, and pretty much everything else have little to no bearing on your strength in combat. I know that right now you earn a buff to endurance for total level of girls but it really is negligible. For me it adds about 5% to endurance and endurance isn't exactly the most important stat in determining victory in combat.
Proposal:
1) Add a second buff based on affection upgrades that influences other combat stats (attack).
2) Consider making buff for total level stronger.