I actually think Northern is the more complicated, as it's where all the issues collide.
If you recall the start date of the current franchise was delayed because a date for the closure of the Oldham Loop hadn't been committed to.
Then during the franchise there's been revisions to allow:
1. Additional Manchester-Preston services (to fill the gap left by the withdrawal of the Virgin services on that corridor)
2. New Nottingham services.
3. Additional Stoke services.
4. Additional Manchester Airport and Chinley services to relieve pressure on South TPE services.
5. Acquiring additional 150s and 158s - some for extra capacity, others to directly/indirectly allow 2, 3 and 4 to happen.
6. Temporary acquisition of the 180s to allow for a delay in the 150s being released.
7. Acquiring additional electric carriages.
8. Changes to Stalybridge services on more than one occasion.
9. Starting/terminating some peak time Mid-Cheshire services at Stockport and withdrawal of a morning Crewe-Piccadilly stopper due to Virgin VHF.
I imagine the next franchise will be similar. There will be changes written in to be done alongside TPE changes but the dates will be subject to change.