I agree that England and HRE should be more fractioned. This is hard though, since the likelyness of them joining a campain depends on their relationship with the marshall, rather than the relationship with the liege (I think, havn't checked in a while).
EUREKA!!! Ok this quote gave me an idea that I think would work for those larger factions and shouldn't be too hard to impliment! Instead of splitting up the the larger factions politically or territorially, just split them up relationship-wise.
For example, the lords of England could be split up into 3 'sub-factions' that have good relations within themselves but poor relations with the members of the other 2 'sub-factions'. In this way, whomever is the marshal of England will be able to call up only around 1/3 of the total faction's armies to campaign. The King would not belong to any one 'sub-faction' so he might show up to a campaign regardless. Plus this does not really subtract from their overall power as the non-campaigning lords would still be defending their lands, and if they are militarily successful the new lords would increase the number of lords likely to campaign. This won't detract from power so much but might slow their offensive strength a bit, and all it would require is changing the relationship numbers between lords.
You could make as many or as few sub factions as you want, and of varying sizes to mix things up. Imagine a faction with 2 strong sub-factions and one smaller sub-faction. While one of the members of the strong sub-factions is marshal the faction can make some good gain, but then a marshal from the weaker sub-faction happens to be appointed and they start losing ground. It would make wars more dynamic and manageable.
What do you think Cruger? Would that work? Do you like that idea?