Welcome to the Org, Sir Louis.

I am basically playing with the houserule you suggest in my current defensive Byzantine campaign, though not quite as extreme as your idea: I chickened out and let myself blitz the nearby rebels at the start, but I only take settlements from the other factions when the Council orders me to. It does make the campaign much more challenging, as it forces you to simply soak up wave after wave when the AI attacks, rather than immediately going on a counter-blitz after a major victory and snapping up all their territory while their forces are depleted. However, I think the Council missions dry up after a while, I haven't had any in about 50 years, and since the AI cities always seem to grow faster, I am now starting to get badly left behind in terms of economic growth. The last mission I had was from the Merchants' Guild, wanting me to send a diplomat all the way to England!