main tinsel
All credit to @unborn bronze
As an example, I’ll be using the seasonal vanguard update format.
Starting with Madoka, let her be a vanguard in the banner and be primarily based on hyper buffing teams with potentially a decent nuke and sub dps.
Let Homura be the secret unit obtained after getting every other unit in the upd evo’d (like Erza), include the obtainment of Madoka and her memoria since Homura ties in with Madoka (unneeded but would be nice to have it tie altogether and may feel rewarding and have an incentive to actually finish update, rather than an optional obtainment). Homura secret should only have sub dps with great time related abilities (revert status of debuffs on self and allies with global cd and timestop for example).
Now make it so you can't use the two together at the same time.
If you use Madoka as a unit itself, she remains as the vanguard in your team, BUT if you use Homura WITH Madoka, then a Homura (WITCH FORM) boss spawns the moment you place down Madoka, once the witch form is defeated, auto-delete Madoka and turn the Homura into Devil Homura (vanguard). Allowing for incredible cc abilities unlocked with this form + great dps at the cost of losing an incredible buff unit and a potential unit slot.
This would make sense for them both as a kind-of duo unit, taking inspiration from Madoka’s story and existing units involving team based sync or the risk of sacrificing units (AOT Units and Griffith). Not to mention how many people LOVE Homura as an ATD unit and the Homura sequel movie FINALLY nearing its release.
Unit elements:
Passion for Madoka vang
Curse for Homura secret
Homura Devil form can either be curse or cosmic