We know that Tirek escaped Tartarus around the later part of Season Two, and decided to show himself at the very end of Season Four. So I'm wondering, what was he doing during that long stretch of time? What would he have done, had the Changelings won? Or if King Sombra reclaimed the Crystal Empire? Are there any fanfics about this?

4202903
Reading Better Lairs & Landscaping magazine, clearly.

4202903 He was probably hunting for food in dark alleys. If the changelings had won, there would've been nowhere to run and hide; he'd be in a pod. If Sombra got the Empire... he'd probably invade Equestria and, unless Equestria wins, Sombra will impale him with CRRRRRYYYSTAAAALLS.

4202903 my geuss is that some one had help him escape also help what ever villian that was in taturerus but then Tirek decided to take matter in his own hand in season four.

Who's to say Tirek can't drain magic from changelings and shape shift into a changeling himself? As for King Sombra, he does utilize what Tirek would ultimately want. Alicorn Magic.