The only the essentials to me are MIDI and stereo output. Stereo inputs and fx loops are nice to have but not essential.
Otherwise to me the important thing for modelers is to do something different. Have some angle that makes them more interesting than the competition. Fractal has e.g their advanced tweakability and huge list of models.
If everyone has the same staple models, cabs and fx, the same Helix-ripoff UI and a varying number of footswitches that do about the same thing as another product, then none of them are really that unique products.