The more I think about it, the more I want something like an HX-DI even more than an HX-LT. Similar setup and function to something like a GK Plex, EHX Battalion, etc.
Pipe dream mode on.
1) Amp and cab sims only for models. Maybe EQ and/or Dynamics (compression) models as well (both the Plex and Battalion have compression). Gotta be careful about cannibalization of the rest of the line, but I don't think adding compression is too much. Utility models only.
2) 1 line input, 1 output, DI/XLR out
3) USB audio interface - as the
de facto last in line of your chain, I think this makes sense
4) Tuner, obviously
5) You probably need 4 switches. In patch selection mode, 2 for up/down, 2 for patch selection. In play mode, 1 for Tuner, 1 for Mode, 1 to bypass the amp modeling. Maybe the 4th is for cab bypass, or compression if available.
6) So at most 3 models running at one time: amp, cab, and compression. Or 2: amp/cab combo and compression. Or 2 with just an amp and cab as separate blocks. Or 1 with just an amp/cab combo.
7) Not sure about the screens. Probably no scribble strip, just one small screen for menu navigation and patch selection.
Even if you take the HX and add an HX-DI, you're still leaving out a bunch of stuff from the LT and Helix (expression pedal, some screens, a bunch of I/O), so I think you can make a case for it not being too much of the main Helix's functionality at too low of a cost, but...dunno.
(Cough...
@Digital Igloo ...cough)