I'd make it as a 2-stage star in a Companion-focused Champion sub-constellation under Craft.
That sub-constellation should allow us to increase their stats (Health, Damage, Resistances, Skill Cooldowns, etc.) even further than what we have with Companion gear, Telvanni Efficiency and Shared Burden sets.
Can we first get Serana? Because lorewise it would work, she only gets locked away at the end of the second era. Also who doesn't want a vampire companion?