Suggestion: Fields break after a given amount of time
I suggest having fields "break" if they've been up over a certain amount of time. Once a field breaks, then all links to the anchor portals of that field will be broken. The resonators on the anchor portals will be unaffected. So, the owning team will still hold those portals. However, the anchor portals will go on cooldown and cannot be linked to again by the same team for 24 hours. Although, if a portal on cooldown is captured by the other team, then the other team can link to the portal immediately.
The change would allow coordinated teams to show their power by creating large fields. However, it would also keep the game fresh by causing fields anchored on long defended / recharged portals to go down, essentially resetting the game for everyone under the field.
I suppose an exception could be made for fields primarily over the ocean, as doing that requires quite a bit of coordination, doesn't affect a lot of people, and is an amazing feat.