Stereo input/processing for me. I always aim to run a single instance of a plugin on a stereo buss. I hate managing a left and right instance of things.
I like to run NAM boosts into NAM captures so dual slots would be cool.
Both of these are great shouts.
I feel like the main reason I like the OG NAM plugin (even when having to use it inside a wrapper) is because its slick and doesn't have a load of filler that I won't use. I get why people might want to have some FX contained, but if they aren't precisely the ones you like using then they're just going to get in the way. I'd rather keep it simple but just improve on what the original one doesn't totally nail.
I think a simple "pre" NAM slot would be cool for pedals. Maybe change the tone stack from bass/middle/treble to a tilt control or something you'd find in a tubescreamer style pedal.
Input and output calibration for all blocks so you can mix and match pedals, preamps, power amps etc easily.
If we're getting fancy from there, maybe a REALLY good tuner built in that can work at high frame rates. I like the Submission LockOn tuner because it displays strobe and a needle together and I think that would make a solid template.
The IR stuff looks fancy but I think I'd probably prefer something with moveable mics to be its own plugin, and just to have a more basic IR loader here.
If the plan is for a bigger/deeper plugin, then I think a proper file management browser for NAM models, presets and IR's would be great.