If repeat FP riders are an issue, you would think Disney would be able to track who has multiple FP reservations for the same attraction at any given time.
For example, MMRR just launched on FP+ today, they should immediately be able to know who booked it more than just once. My guess is that it might not be significant, otherwise, it's easily correctable and I would question why they don't already do something to curb it.
I think they could easily cap all guests to just one FP (each) for the most in demand rides every 30 days. I mean, we know the rides, but to mention them, FoP, Ratatouille, MMRR, RotR, MFSR, SDD, SDMT, possibly a couple others. What I'm talking about is not a guaranteed FP to these rides, but a guarantee that you can't ride them twice within 30 days using FP (and the same magic band, since some people swap bands; a loophole). Then, if someone wanted to ride it two times or more in a 30 day period, they can do standby.
For example, let's say I got MMRR on March 4, I wouldn't be able to get it again until April 3. I could still do the other T1 attractions though until I've used them, so think MSFR or SDD at DHS (both of which would automatically block out availability after they're used, for 30 days). Other rides and attractions would remain available as they currently are and you would still be able to book three available attractions per day.
I believe something along these lines would definitely make it more fair and give more guests the chance to experience the attractions. You'd think it would be easy since it's all electronic already and linked to your Disney account. I know when passes are blocked out, they won't let you book, for example, this isn't much different, just specific to attractions. I'd require some technical work, but they're more than capable, I'm sure.