But people with a high BS have the bonus of being able to collect lots of regency already. In RoEII I tried to increase my BS as much as possible because the benefit IMO was worth the effort. To tie stability to BS would IMO make it so powerful that it would move from something I would do whenever possible to something I would feel I needed to do every turn. Is that desirable?
I'm not sure I agree with the outcome, firstly a domain of 60 is pretty big (although the P&H for turn 1 shows 5 players are at it most are well short), secondly you can sidestep the stability penalty by use of vassals. So it could be argued that the mechanic is designed to drive creation / maintenance of vassals rather than encouraging high bloodline - getting most bloodlines up towards 60 would in practice be extra-ordinarily expensive, it would be much more cost effective for Joe with bloodline 30 or 40 to spin off a minor vassal or two when his domain grew to 60 than to spend 20 turns or so of regency income solely on raising his bloodline.
You are probably right that in this setting you would hardly see people invest that much regency on BS since we won't have such big economies as some realms had in RoEII. But there is still a huge difference between looking at a domain at the beginning at a campaign and noting that only few will be affected in the start of the game, to knowing how the holdings are divided ten turns into the game. And spinning of a vassal, while keeping your stability up, will cost you GB income, and you are not likely to gain more than a few RP (and hopefully a somewhat loyal ally)
I like the limit of 60 better than the limit of 40, and don't like Brandon's reading at all. 60 is large enough that most regents won't be bothered by it. 40 will hit some, and since you likely drop from +1 to -1 stability then it stand to reason that you would try to circumvent the penalty, either by increasing your bloodline or by creating a vassal... And yes, creating a vassal first would be fastest in almost all cases.
But as soon as your RP income is larger than your BS, and if you have more holding levels than BS, then increasing BS is a sure way to more power. Linking BS to stability will then make it a sure way to keep your domain effective while granting you extra resources to do your actions. It is a double bonus: no penalties and RP to use in influence.
Vassals come with the risk of rebellion, they have their own agendas and they cost you money. The risk you incur by increasing your BS is next to none, if you have saved up a regency buffer.
Yes, there will always be faster ways to amass power, but IMO none will be at as low risk as just increasing BS to gain higher regency and offset negative stability modifiers for guilds and landed rulers.
Since wizard and temple domains have spells as well as normal actions to use their regency on, I see the variant as a "hidden bonus" to domains that doesn't cast realm magic, as they will be more likely to benefit.
I know that, in effect, linking BS to stability creates options for keeping your stability high.. A purely static border for domain size only foster vassals, whereas a dynamic border linked to BS gives the choice of increasing BS. But the option is only available to the few, and as such has the potential to be a game balance breaking option.
I wouldn't object Garth's reading of the variant being implemented. Only because, as you note yourself, there will be few if any who are affected by it.
But still, I would like the DM's to note that by linking stability to BS we give an extra option to those few lucky realms who are large enough.
And ask if it isn't better to have a system that doesn't link stability to BS and IMO is more fair for all?