For SP, the improved AI is worth it.
I agree that diplomacy is great. For a new mod, would it not be easier to make a mod for Diplomacy and downgrade for Entrenchment?
Depends, if your mod uses the diplomacy research trees the reverse is true. Where as if your mod is made for entrenchment it won't affect the diplomacy research side of things upgrading it. Other than that, I would think that it would be the same amount of work which ever way you go.