My own armchair proposal for that problem is to relax the perimeter rule in some fashion, but tie any additional beyond-perimeter flights to maintenance/addition of flying to small markets, and/or mainline aircraft (for slot efficiency at the constrained airport) to medium-to-large markets (e.g., tell UA they have go to all-mainline at DCA if they aren't going to add more diverse service like say DL does). Not saying that is perfect in any way, but it seems to me to be a logical starting point that covers all the bases between access for beyond-perimeter cities*, while maintaining small-market services, and as well trying to entice the airlines to maximize total available seats per slot pair for efficiency's sake (understanding that from the airline's POV, some markets only work with 76-seaters or below without some sweetener somewhere).
*: Part of my thinking is that the beyond-perimeter program does face the risk of everybody jumping onto say LAX, and likewise say WN being able to presumably garner a tasty premium on the AUS flight. As such, incentivizing new service (resumption of SAN seems like a no-brainer, SAT seems logical, maybe an SMF or an SNA or something, LGA-style weekend service to BZN in the summer) is of course always on the agenda with any new exemptions, but likewise, you want to also encourage competition on the likes of AUS or LAS (SLC and PDX for instance might not make sense for anyone else, as opposed to LAX as it is now, and maybe SEA or DEN too down the line).