I think that the bastion system should primarily interact with the downtime system. I can see it as being used to help organize downtime activities and tie them together with lifestyle expenses.
I am picturing a modular system that lets players and the DM build their base over time. I'm picturing 4 types of options to pick.
1. The type of base. Offer a wide list of different types of bases, each with its own benefits, drawbacks, and new options, and hirelings.
A Tavern, an entire Village/Neighborhood, a Castle, a Cave, a Hidden glen, a Ship(Normal and Spelljammer), a church, a school, a guild hall, Ancient Ruins
2. Location of base. Can add and restrict the type of options players can add to base.
A City, The countryside, The Forest, A Swamp, The Mountains, Underground, a Ruined City,
3. Options for the base. Provide options that are useful for each class, as well as for various skill/tool proficiencies, and ways to gain and manage different hirelings.
A wizards lab, a temple, a thieves den, stables, troop barracks, a jail, various workshops,
4. Adventure Hooks for the base. Unique aspects about the base that can add spice and lead to adventure.
A portal to the shadowfell/feywild, Its Haunted, Used to be owned by rival who wants it back, Under threat of invasion, Used to be owned by someone famous