As has been said countless times here already, structure spam is out of control. Full Disclosure; this idea is based on one shared with me by an alliance mate, so not completely my own.
What I want to center around as an ‘explanation’ for this system, is the idea that too many structures distorts the gravitational balance of a system, thereby forcing us to adhere to restrictions preventing us from causing damage to star systems.
To do this, restrictions can be placed both restricting how many structures can be placed in a system, AND how many structures can be placed on one grid. For continuity’s sake we’re going to use the current mass of structures in our requirements (50 bil kg for XL, 10 bil for Large, 3 bil for Medium). These restrictions could only apply to new anchoring structures, or CCP could force an unanchor of non-compliant structures with fair warning (and reimbursement of original rigs).
The grid based restriction would be a hard-set ~76 billion kg, or the equivalent of 1 XL, 2 Large, and 2 medium structures. This would help to de-centralize things from one-grid, and remove the ability to view all important structures in a system from one area.
The System based restriction is a bit more complicated, and is different on a per system basis. This restriction would be ~13 billion kg, multiplied by the number of planets in a system, plus the single grid limit (~76 billion kg). With the most planets in one system being 18, this would allow a theoretical maximum of about 310 billion kg, or 103 medium structures, with the average being closer to about 180 billion kg (with 8 planets), or 60 medium structures. This sounds like a lot, but when you take into account Large and XL structures you hit this limit pretty quickly. I realize this also makes it impossible to place moon mining facilities on every moon in some systems, and honestly I don’t have a problem with that.
These numbers can be tweaked of course, but my hope is that this idea can be made viable.