Lots of good recommendations. I'll have some reading to do during the holidays, see what works well and what doesn't!
I'm open to tinkering with that! Any recommendations?
Savage Worlds had this via the Tattered Banners companion for the 3rd party Beasts & Barbarians settings. It has content like war band creation, additional rules for Mass Battles and how to manage war bands. It also contains some GM specific rules like how to support war bands in a campaign, as wells as conquest points campaign system. It was written for the previous Savage Worlds Deluxe edition of the rules, but there'd be little problem using it with SWADE. There's some contents that's specific to the B&B sword & sorcery setting, but not too much - mostly setting agnostic....Savage Worlds also handles groups of followers well, though it doesn’t really feature in any of its settings.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.