I'm going to regret even following up on this, but....
I mean, it's one thing to have the Eldar out there fighting Chaos and rah-rah, that's great. But let me sketch a little future for you. The lizardmen wake up from their stupid war, make peace, and decide they should try to rise to greatness as a race and take their place fighting chaos in the greater galaxy even without thing 'missing species' of Slaan they no longer have. They build a spaceport on their homeworld, Avernus. They send out ships and start establishing colonies in local space... the areas controlled by the Imperial Trust? Or maybe they settle inside already existing human colonies? And even if they're trying to be "nice", they're stronger and longer lived and have at least some Old Ones technology.
They're going to dominate whatever space they're in if they leave behind their current war, and the space they're in is the space we're in! They're naturally going to expand outwards from their effective 'homeworld', which is Avernus. They may be "out in the Galaxy fighting" but they'll also be in our backyard.
I mean, it's like the Krork. It's great that the Krork are back and fighting, but I wonder what life is like for human worlds in Krork territory. At the very best, they're second class citizens who live in protectorates, allowed to maybe govern themselves but certainly restricted to space travel where and when the Krork allow, fighting wars as the Krork direct. The lizardmen go out into the galaxy, and someday the Imperial Trust may very well be nothing but a lizardmen protectorate. Avernus will be a lizardmen world, where they maybe continue to allow humans to live and govern themselves... basically, humanity turns into the Quartok in this situation.