[What Are Some Otherworldly Chord Progressions?]

Using chromatic mediants, sus chords, and augmented chords is the usual, but you can always make them sounds fresh.

[Creating a realistic impression of depth in stereo mixes]

eh, almost nobody here will need to use surround formats.

 

Those tips are good, I'll keep doing things with the traditional tools for width (hard-panning, chorus, reverb, ping pong delays, synth unison/double-tracking).

I think the overlooked thing here is that if you want a wide mix, you then need a huge contrast between the center things and the wide elements. The biggest killer of that is usually reverb, so here's a tip to combat reverb making your mixes less stereo:

Dual-Mono Reverbs, or simply separating the two Left & Right channels and putting the same reverb on them, is a technique that will preserve the position of your panned elements exactly where they are. No more 'stereo image blur' with that.

[DAW based on sheet music?]

I think this was kinda popular in old-school softwares (but less than Tracker-based input), so look there (but don't expect newest VST support and modern things).

 

Try Harmony Assistant, it even has its own computer singer.