MadBomber developed an outstanding node theory for his WIP campaign which I think works really well, and I've decided to use in my campaigns too.
From Freespace canon, it's implied that nodes range in stability from completely stable to completely unstable. One can assume that there's a certain "threshold" of stability that nodes must exceed in order to be traversable by GTA/PVN/GTVA ships. (One can probably also assume that this threshold is much lower for Shivan ships.)
MadBomber adds to this that there are "phasing nodes" whose stability changes over time in a regular (or even irregular) cycle. So when the stability moves above the threshold, they can be used, and when the stability moves down again, they cannot. The GTA and PVN made extensive use of phasing nodes in the Great War (and probably also in the T-V War) but the GTVA is much more cautious. Thus the FS1 node map (seen in the command briefing animations) shows phasing nodes while the FS2 node map shows only purely stable nodes.
In my own notes I've added a corollary that through some esoteric laws of subspace, phasing nodes cannot form on their own: they depend somehow on the presence of a fully stable node in the same system. Therefore if all stable nodes in a system are collapsed, the phasing nodes are also collapsed via chain reaction. Before the end of the Great War, the Sol-Delta Serpentis node was the only stable node in the system: the nodes to Beta Aquilae and Deneb were phasing nodes.
I haven't decided yet whether the phasing nodes will re-form themselves if one of the stable nodes is repaired, or if those phasing nodes are lost permanently and different phasing nodes subsequently form. Either option offers interesting possibilities.
