Arguably a megastructure would by definition need to be on such a large scale that it can’t compare to thousands if not millions of said normal stations. I can see your reasoning, but I believe there’s a significant size difference.
Let’s assume in this case the definition of a megastructure to be at least the size of a star. This means that, if we take our solar system as an example, a star can fit about 1,300,000 Earths. To illustrate the sheer size, a cathedral (120 m height) is merely 100,000 times larger than an ant (1 millimeter length). I think it’s safe to assume in comparison to a megastructure, normal-sized stations/ships (which I believe the current average to be at about 5 km, just a hunch though) are quite literally ants. It’d be like applying car laws on atoms. Yet even though these normal-sized can be spammed into millions, they might only equivalate to just one megastructure. In essence, there is in fact an enormous size difference at play here that perhaps our minds can’t truly comprehend.
To that extent, I don’t think large capital ships over 100 km or even 1,000 km should be considered megastructures. Because they too are not comparable by size to a megastructure. As such, because of the sheer size of a megastructure, they’d have to be stationary for they are too large for out star nations to move around. Nor would power creep have any real means here because of the giant size difference. Or at least, that’s my view on it.
In short, megastructures are in fact so large (or at least the definition should clarify the size) that can’t be compared to normal-sized structures. The latter are already capped in a way, because each plot has a maximum number of planets, so it wouldn’t need any additional rule I believe. Because of said immense size difference, there needs to be a cap to ensure a power balance between star nations.
Technology-wise, maybe. But I believe we should and can still leave that largely open to ensure a large enough freedom.
As for the sub-plots, yeah it kind starts biting us in the ass now, doesn’t it? But overall I think the cap should apply to the larger plot and different subplots can discuss this internally I believe. That seems like a bargain to me.
Similarly, if a star nations makes territorial gains, I’d go to say that it becomes a modified plot and the previous rule doesn’t apply to it, because that rule would only apply to the original plots. Ergo, document each plot very well OOCly.
Edit: Anyways, I’ll probably not be able to join this discussion for quite a while because of RL priorities.