Something like the DDMF Plugin Wrapper would be amazing (
https://ddmf.eu/metaplugin-chainer-v...s-aax-wrapper/). Rather than a linear plugin flow, native support for a graphical environment where signals can be split, duplicated and remixed independently would really take Reaper to the next level.
This would allow users to, for example, trigger a snare sample using a live recording, mix the sample in with the live snare, process both independently, and add a common compressor / reverb over the top, all from within a single track.
Or, on a mix bus, split out a number of parallel processing chains, and mix them together again. If you hit a sweet spot, you can recall the fx chain without having to import aux tracks.
This would keep the arrange and mix windows more clear and streamline the work environment. It would also allow for some more creative mixing techniques to be easily applied.