Second guild

House rules: Second guild

Historically as the years drew on and the Renaissance began there was an explosion in the size of a class formerly almost unknown - the middle class. Suddenly instead of just the nobility and the priesthoods with wealth, there was another sizable group of wealthy influential people - merchants, artisans, etc. In typical BR games guilds are simply another holding, no wealthier than priests and somewhat poorer than rulers (barring trade routes at least).
To some this situation is historically anomalous - nobles went cap in hand to bankers for loans, wars were financed as much if not more by guilds and wealthy merchants rather than nobles and temples. While holding types are quite blurred (a true medieval church could easily hold province, law and guild levels not just temples) a gap can be perceived.
To 'fix' this gap the income of the middle class - i.e. the guilds - may be increased by doubling (or even tripling) the number of guild holdings (and guild domains). This creates vast numbers of wealthy merchants reflecting a powerful class in society, it also allows nobles, priests, etc to have small guild holdings to reflect their economic activity without restricting the power of guilds overmuch.

From a meta-game perspective although it would be administratively simpler to simply double the income of the existing guilds, that causes certain balance problems - guild holdings abruptly became far more valuable despite having the same cost making every ruler demand guild holdings and competition severe. More importantly existing guild regents become far more powerful than other regents (as the guild domains are typically designed to be on par with other domains, perhaps slightly inferior to landed domains) and so on. It is worth noting that some costs (court costs, number of actions, maintenance of key structures, etc) are almost fixed, with the result that doubling income results in more than doubling the 'surplus' available for actions and troops.
Doubling (or even tripling) income does not necessarily need to increase regency income - it could simply reflect more wealth in the economy. Adding a second holding on the other hand will inevitably increase the amount of regency as otherwise the holding is highly anomalous.

A second guild is easiest to explain mechanically if it is identical to a standard guild. DMs may want to consider whether second guilds can possess trade routes or not depending on the complexity they are happy with in the game.
Distinguishing between guild types thematically could be done along production types (production of raw materials vs worked goods, internal vs external trade) or any other distinction desired.
Effects which compare relative level of holdings (say rights to raise troops, levy taxes, provide bonuses/penalties to actions) will see guild levels up to double the power of 'normal' holdings under the second guild approach. This may be perfectly satisfactory, or could be dealt with by halving guild levels for that sort of interaction, or by considering the two (or more) types of guild as totally separate holding types.