Okay, I'm probably your exact target demographic - I have about 1,300 hours in Rocksmith and I'm obsessed with it, but often find myself wanting more "pro" features like Guitar Pro has. So hopefully you'll find my feedback useful...
On the tab interface:
- Seeing a blob of 6 numbers come isn't intuitive enough. The strings REALLY need to be colored. You should be able to get away with using the same colors as Rocksmith, as there's generic sets of strings using those (https://www.guitarcenter.com/DR-Strings/Hi-Def-NEON-Multi-Co...) - or at least make them configurable. It's hard for me to tell which string is which in fatpick.
- Bar lines for the beat/measure changes would be helpful.
- Chords saying their name above them would be a huge help - "G" is much easier to parse quickly than "355433"!
Practice mode:
- Just adjusting playback speed isn't enough. To be actually useful, fatpick needs something like Rocksmith's Riff Repeater, where you can select a section, loop it at a selected speed, and have it increase 1% each time you get the notes right. In fact, here's where you can outshine Rocksmith, since Rocksmith only lets you select entire sections, whereas you could make it so you could really drill in on notes
Audio:
- Being able to play the audio file instead of just guitar pro-style midi would be a huge bonus. Don't make it required, so you can still just import a quick GP file to play, but for curated tracks or imported Rocksmith psarcs (see my comment elsewhere about psarcjs), the real audio would be great.
- Even better... multitrack audio! Just play the song .mp3 if that's all there is, but there's plenty of multitrack ogg's sourced from rock band / guitar hero floating out there. For these songs you could play with your chosen instrument removed - IE, pick a song with a multitrack ogg, pick the bass part, the audio engine wouldn't play the bass part but would play the rest.
- Some kind of effects on the guitar would be nice. Using an AxefxIII and Loopback I can mix in real guitar sounds with the game, but most of your demographic won't have that ability, so being able to pick an AC/DC song and have basic distortion effects will be a must.
Anyways, after playing with fatpick for 20 minutes I'm going to go back to Rocksmith. Will check on this in a month though and see how far along it is - there's a lot of potential here!