It is a good idea. I want to get the peasants to be a bit more interesting, so will be looking to add a set of buildings based around manipulating them. Peasants actually have their own politics and stuff, which can be spread from city to city, but it's very under-developed for now, so needs working on.
Having them escape from civilisation to set up brand new cities could indeed be neat. I like the idea of you inviting the peasants into your lands to offer them safety, in order to allow them to grow and build into a stronger and more capable fighting force. Maybe you could provide them with weapons and stuff, to make them stronger (currently they're only 25% as strong as a normal army, per person). I'll look into it, to see what works and what can be done.