I have been thinking about how to 'force' source domains to interact more with the other domains. I think there are a few ways that might help spur this.
*Relax the rules of Advantage/Hardiness in relation to Source domains: Allow Law/Temple/Guild domains to confer bonus to create & rule actions of sources. Allow Sources to affect agitate, rule province, contest holding (all), etc. I still wouldn't allow domain to occupy source domains, nor allow source domains to be affected by armies.
*You might consider giving virtual trade or guild holdings. Giving them a limited source of income will not tip power scales very much, and will give the domains reason to be concerned about what is occuring in the non-source domains.
*Giving well establish Source domains hidden and non-hidden manor, guild, temple and/or law holdings. This holdings would not represent neccessarily the source domain attempt to enter those arenas in force, instead it is represents close ties the source domain may have with various power structures. Taken to an extremely, you would get the source domains that directly rule several holding types.