"We've disabled having the monsters attacking them as it was incredibly tedious.
We are looking instead at having those outposts give you bonuses in their area of effect."
I felt compelled to do a mock-up. (Even if I think the current defensive line of upgrades can be salvaged rather than scrapped). I'm sure there are lots of other potential outposts... using ideas from Tower Defence games like Gemcraft you could have lots of interesting Tower/outpost effects:
Poison (any DoT). Preferably lingering for several turns after leaving AoE, possibly as a bonus for allying with Swamp neutrals.
Lightning (splash/chain elemental damage, fun particle effects. Useful against enemies immune to non-magic attacks),
Black/Blood-bound (greater effects based on number of attacks/kills/resources like 'ruthlessness'),
White/ Mana(pool)-bound (greater effects based on your current mana, and nice resources like 'hope'),
Mana stealing (each attacked enemy gives you power, more when attacking mages or magic troops),
Armour tearing (each hit debuffs the enemy, befuddles them or otherwise mods their stats). As well as lots of other ideas more loosely inspired by many games or even previous elemental games:
Amplifiers (makes magic stronger in AoE, perhaps lowering terraforming costs or increasing effects/rewards),
Dampeners (makes magic weaker in AoE, interferes with SK spells and provides resistance to hostile terraforming),
Consulates (spawns/attracts neutral NPCs, may offer quests),
Stables (increases moves, or recharges moves of armies in AoE - allowing for nice supply lines without the super-buff of roads x400% movement speed.)
...there are as many good ideas as there are fishes in the sea. Sometimes there are so many the shoal seems daunting, but each is tiny and harmless. It's up to the devs which little fish they like the look of.