"With regards to the faction system, you could use the basic one that everyone uses (Y'know the Republic/China one) but replace the factions with the House of Lords, the House of Commons and perhaps the Monarch? The HoL giving reduced noble rebellion chances, decreased cavalry cost and perhaps other benefits, the HoC giving increased trade income/power along with increased manpower, perhaps other benefits. The Monarch could give less stability costs, greater troop morale and +1 tolerance of the true faith?

Either way, it'd add a second dynamic to the mod. Not only would you have to chose a party, but also a faction in the system of government."

While giving benefits via faction is a nice thing, my concern is that it would give too much benefits to England/Great Britain because parties already provided some of that. For example stability cost reduction is provided by Tories and their equivalent while Whigs provided increased trade power/efficiency and a tolerance bonus. It's possible that I could move them to the factions as each parties has multiple modifiers but I'm not sure. There is no cavalry cost reduction in any of the parties, however, so that's kind of doable.

Another option would be simply to leave factions without any benefits but use them as trigger for events. For example if House of Lords has Tory majority but House of Commons already have Whig party modifier assigned for the country, then perhaps there could be some kind of political drama event. In fact, I already have plans for events dealing with bills passing or stalling in Parliament, like the one dealing with Catholic Question and other historical issues.