Probably not on the drawing board, but I've got a request for this model: make it better for live use (better than the 500X, even, IMO).
Add processing power and change the AMP pedal to FX"0" so I can have 2 overdrive/distortion or, my preference, compression + od/distortion capability. Maybe even make it a reverb on/off control, too.
For anyone that wants to bypass the AMP, make it a switch. I just don't see that as being something to change in a live situation, where the floor switches are really needed.
Oh, and use better paint/decals/whatever so the labels hold up better.
As it is, the 400 is just about perfect for live use where you've only got a few different settings and effects, and where, honestly, the finest level of tonal perfection is wasted. "Close enough is good enough," and less time tweaking. But, the lack of compression *and* distortion together is a gap, and the one thing that has me staring at the used 500 market. But, I'd buy a 400X before one of those if it had patch compatibility with the 400.
Oh, and how about a MONO switch for XLR out. That seems to come up all too frequently here.
As it is, I don't see the distinction between the 300/400 making sense. Keep the 300 at a bit lower price to combat competition at the $200+ price point, but make the 400 something slightly different. My $.02, anyway.