In my opinion, making it simpler would be a good starting incentive. 

 great question, many relay providers are dabbling with paid/subscription services.  They improve the functionality of the relay in some way (anti-spam, improved performance, high availability), and in return users send them some sats to use the relay.  Relays could also be stood up simply if you need more control.  But to your point, there are no "rules".  A free market is emerging and the market will decide what type of relay is valuable.