Assume that Ultramar in 41st millenium with 500 planets and 15 trillion people and thousands of warships is transported to ME era and Ultramar is lead by Roboute Guilliman and he has billions if Imperial Guard and a quarter million Ultramarines. Assume Ultramar is transported to ME-verse around the time of First Contact War.
How will this change the timeline?
I'm not sure 15 trillion people is actually enough for ultramar, the realm has multiple hive worlds with it that can easily hold that many or more people per planet, so they're actually going to be significantly under-populated per the norm.
It would also make sense to, instead of bloating the 41st millennium era ultramarines to a massive degree, to just have there be two or three hundred ultramarine descended chapters in the area (which might already be the case, I'm not sure exactly where all the ultra's successors are but a bunch of them took stewardship over parts of the former realm of ultramar).
As for changes to the timeline, Ultramar takes over the galaxy without any substantial effort, creates the IOM 2.0, but with more of an orginal flavor Rome theme than the quasi HRE thing the Imperium does.