A simple survey with options (similar to TSS estimator) would be nice here right after completing a workout. Having a simple number here would also make it easy to compare RPE for past completions of the workout (or workout variants) to know what to expect when looking at past rides and preparing for the workout ahead. Furthermore, it would be useful to aggregate these surveys on a weekly or so basis which would help inform (with avg TSS, IF, etc) whether another FTP assessment or adjustment is in order.
I know there is a notes option for rides, however, it takes quite a bit of effort to type something in especially when not on a computer, when most of the time all I care about it how hard I thought the ride was. Notes are also difficult to discover and would not help trying to compare RPE of workouts over time.