Tachy Bunker

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

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.

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.