As an idle modeller, most of my "what if" scenarios are just reliverying:
1. LNER bought a few odd bits of kit to test out on specific routes - so I have an Apple Green 14xx on a quiet branch line.
2. Bulleid stayed at LNER, and took over from Gresley - so we have the Dales Class of air-smoothed light pacifics...
3. No Nationalisation - so Deltics and Brush Type 4s in Apple Green...
4. No Modernisation Plan - so steam locos in BR Blue
TBH I've only reached stage 2 so far (and not very far through that), but it's 3 I want to do most.