'One simple option is if you lock we lock aka a small eg <1% of your lockup goes to the platform in the form of a lock. I.E. if you lock up for 30,000 blocks our fees is locked up for 30,000 blocks'

Works for me, the incentives would be there for recipients of fees to keep making the app better for users, to encourage increased usage and easy onboard of new users to grow the app and grow the fees.