Had the similiar idea lately, but instead using doofers for that by simply adding a general dry-wet-slider to the doofer device. This plus virtually free placeable send tracks (routing possibilities could stay the same, only for better visualization and grouping).
Better organisation of send channels, grouping etc.
Generally allowing fx with more than one stereo input or output, with added routing like via send/receive devices (and each fx input would be a port to point send devices to, each output can act like a send device).
Having “doofer” kind of devices that work like an instance of the instrument fx stuff. Add many “lanes” of parallel fx processing, make parameters accesible/automatable like you’d do with instrument macros, have each “lane” point to different output options that will appear to the main mixer matrix like an effect with multiple outputs described in 2)
I think this could be done with quite some work, but without messing with the current ui concept too much. Of course this fully modular synthesis/routing shit like having lots of little boxes with ports to connect with virtual cables on a 2d canvas is maybe the most flexible, also in terms of keeping overview over complex routings. I always feel like current renoise tries to keep shit at the historic level of having racks with stacked fx boxes that are connected to single channels in a hardware mixer, always with serial processing until the final mixing stage. Matter of preference. Real modular routing can enable one to make quite some rad stuff.