Air Lord, Engineering Corps
Here's a quick suggestion I think would be pretty simple to do and would make the campaign much more interesting.
Forward Base: Forward bases are blights that are created by having a military fleet (yours or the AI's) in a blight position near an enemy city.
These blights are very weak in comparison to normal ones but can be upgraded by disassembling your fleet, provided the ships have any (1x1, or 2x) 'building supplies' modules that add upgrading power to the vehicle.
The Building Supplies module is heavy, making dedicated construction vehicles important.
Building Supplies (and therefore Forward Bases) are a city specific locked module.
The Forward Base lowers enemy income and adds it to your own. Sometimes the base will do a raid against nearby cities if it has been upgraded.
Something like that could be quite fun. Or some way of encouraging guerilla warfare against cities, CIA-style. Pay for a guerilla outpost to be established.
In addition to this it'd be a cool feature to allow you to pay pirates.
What it would do:
Bribing Brigands and pirates will automatically upgrade their blight, but you get to tell them what to do for a while (raid cities, steal small ships, etc.)
This helps weaken enemy cities, but if you capture them you now have to contend with the hostile pirates you upgraded as they are now hostile to your newly acquired city.
There is always the risk of accidentally creating a pirate stronghold, which can't be bribed and frequently raids cities, as well as creating more blights and bolstering existing pirate and brigand blights.
Very much my thoughts too. I want to put in the ability to pay off most human blights - so also mad scientists but probably not cultists. You can pay them some amount to make them leave you alone for a while, or more to hire them as mercenaries to attack another player.
But yeah, all that money goes straight into increasing their size.
I also want to add (probably in 9.1 now, I've declared v9 feature complete so that it can finally go live) the ability of monster nests to upgrade to worse versions of themselves.
So brigands become pirates, mad scientists develop their tech further and make worse strongholds. As for dragons and the planned cultists, well... :D