Here are some ideas:
Pay a lower rate to the heavy side when unbalanced (MoDs idea)
Pay a flat amount total symmetrically to each side distributed evenly amongst LPs on that side. So if you give out $1 to each side and there’s 100 nbt on one side and 200 nbt on the other, the heavy side gets 0.5% while the light side receives 1%. (Fixed cost)
Have a target amount for buy and sell separately up to which a fixed rate is paid. When liquidity on one side goes above target, enact fixed cost on just that side.
Rate = MaxRate * min(buy,sell) / max(buy,sell)
Some kind of dutch auction mechanism. However, I think this will be impractical to implement.
Make the operator reward linked to how balanced the peg is. Give some leeway for arbitrage and so on, but if one side ever crosses say 75% of the total liquidity we dock operator reward by 25%, then 50% at 90% unbalanced.