Eh, I think I mentioned earlier that a good way to humble the UM would be to have all their Chaotic enemies decide to gang up and finally crush them them once and for all, as part of a mini-Black Crusade because literally the any thing any champion of Chaos can agree on is that the Ultramarines are a bunch of buzzkills. A good way to reintroduce the Lion would be for the DA and Successors show up in Legion form, all painted black (because no way are they getting to wear Caliban Green after the shit they've been pulling to cover up their secrets TO DEMONSTRATE THEIR UNENDING LOYALTY), fuck up the forces of Chaos, and the Lion meets with G-Man, starts shit with him about how he's finally gotten the Imperium all to himself, and then announces his Legion is off on a penitent Crusade for the shit they've done, and in the meantime save the galaxy's day as G-Man sits around with his thumb up his ass ruling like he always wanted to do. And yes, Luther was forgiven. He was also blammed for his treason.