A small concern... I think this might be a little over the top.
Understand me right, I've not played with this or the previous edition of the rules and I get that this is supposed to make the holdings a little more static and more of a long term investment.
I hope I'm not whining, but I look at the numbers and I think that the small fish (anyone with <20 GB income, <50 RP gen) are gonna get hit very hard by this.
Some examples:
My domain, Tornilen, has a Source (2) holding in the North Fell of the Spiderfell. It's competing against Lord Gorman's Source (4) holding. The magic potential of the North Fell is 8. It's very hard for either of us to increase our holdings.
DDC to rule my Source (2) to Source (3): 10 + 3 (holding lvl) + 4 (gorm's source) + 8 (potential) = 25. In order to make that by taking 10, I have to spend 30 RP, I get 38 each turn. I have the remaining 8 to overcome whatever lord Gorman spends on stopping me.
For Gorm the DDC to rule his source (4) to source (5): 10 + 5 + 2 + 8 = 25. Making 18 RP/turn, he's not ever going to rule that one up, having to spend 30 to take 10, then overcome what I spend to stop him.
I've ignored stability for this example.
Different example:
Barony of Serien, Serien (5/2):
Someone wants to create a Law (0) holding in Serien, Xavier Zenneth controls a Law (4) holding, hardness applies.
DDC: 10 + 5 (province lvl) + 4 (hardness) = 19. The agressor has to spend 18 RP to reduce it to 10, then overcome XZ's defensive spending.
Now... My point is, those numbers look awfully high, unless you've the income to overcome them. GB counts more than RP - they can be spent 1/1 to reduce the DDC. I think that adding the province level / magic potential to the DDC of rule *whatever* actions reinforces the status quo rather heavily. In my example, neither lord Gorman nor Tornilen/SM, is going to get that remaining source level anytime soon.
I'd suggest that we start out with adding ½ magic potential or province level to the DDC and see where that takes us, which is more in line with the numbers you initially suggested.
DDC = 10 + new level + ½ province level is almost as cute
Of course, my perspective is that of a small fish looking at the big boys, but the numbers seemed... daunting, considering the income of my domain and other small realms in the game.