Epibionts: Your species has spent nearly all of its known history living upon the titanic life that roam your world. Where they have gone, so have your people... and where your empire goes, so do they.
No, this isn't unlimited titanic beast armies day 1. Rather, they take the place of normal residential districts, costing food instead of minerals as you "breed" more large enough to live on. Ideally, you can customize a bit what sort of titanic life it is to decide what else is unique about them (like they might be Lithoids that result in one clerk job being a miner job and take minerals, or their body's ability to modulate its temperature creates a "climate" on it that remains reasonably consistent even on other non-native biomes, resulting in a habitability boost) which might add or remove how much housing they create. Your capital might create a titanic beast defensive army instead of normal ones, but otherwise I figure you'd have to wait until you unlock uplifting or some other gene tech before you can create offensive armies (to make them smart/fierce enough to war with) and still have some sort of hard cap (such a 1 per planet, or based on your empire's total population) In my dream version, they aren't just some unique district, but are presentient pops, which you can, upon uplifting, decide to enslave as excellent workers for your farms and mines (though that means revolts will include titanic beasts) or treat them as equals, which might have other bonuses... though regardless I don't think they could be leaders, since they would have to live for millennia to be seen as a reasonable habitat, which would mean they are effectively immortal, and immortal leaders are suppose to be rare.