If you're looking to encourage new carriers, you're going to have to offer even more protection than at present.
While Ryanair pay full commercial charges and Wizzair pay reduced ones, that clearly hasn't discouraged Ryanair from jumping on their routes. So you're going to have to impose higher charges if someone launches on an existing route (esp if it's launched in the last 5-8 years). This wouldn't actually be illegal if transparently and consistently applied.