Who is Galen Marek?
Galen Marek - also known as Starkiller - was the apprentice of Darth Vader. Born on Kashyyyk as the son and only offspring of two Jedi who deserted during the Clone Wars, he was abducted by Vader after the death of his mom and dad (the latter at the hands of Vader).
How would the character be incorporated into the new Star Wars universe?
Have Mikkelsen play him. They do look somewhat alike.
While Galen Marek's previous origin could serve as a great base, there are some elements that would have to change if Mikkelsen is set to portray him.
First, Marek is said to have been born roughly 19 years before Luke destroyed the first Death Star, but since Mikkelsen is 50 years old – and Rogue One is taking place shortly before the events of A New Hope, this version of Marek would have had to be conceived much earlier: most likely even before the prequels! Although, this is an advantage, in the long run.
He could, and should still be the son of Jedi deserters – two Jedi's falling in love and leaving the order doesn’t seem to far-fetched – and it could also add further weight to why Jedi’s are forbid to marry (even if it's an ancient rule).
However, this Marek will never have been the apprentice of Darth Vader.
Instead, I'd suggest that his father, or both his parents should have been killed by someone else, someone who back then they thought was a friend: Count Dooku. It could be so simple as Dooku asking them to join him in his new direction, and they refuse and therefore he kills them.
Unable to receive proper training in the ways of the force, Mikkelsen's Marek spent years training himself with the help of his parents’ journals, but also by traveling the galaxy searching for other force users, so that one day, he would get revenge for his parents and face Dooku. Encountering both good and bad, this Marek ended up walking the line between good and evil and by the time we’re introduced to him in Rogue One, he’s our first encounter with a Gray Jedi.
Considering this Galen Marek had been unable to fully get revenge for his parents (since Anakin killed Dooku), Marek relocated into the shadows (before later finding out that Sidious was the one calling the shots). Then, when Sidious was revealed to be Palpatine, and his Empire was constructing a weapon that could wipe out whole planets - in a time when the Jedi are seemingly long gone - an aged Marek offers his talents to the Rogue One-mission to help prevent this monster of a weapon from being built, but also in order to try and avenge his parents, one last time.
The Possibilities This Creates
Spinoff
A spinoff, or anthology film, showcasing Galen Marek's earlier life as he travels the galaxy meeting other force users, looking for his revenge on Dooku. Possible encounters could be Qui-Gon Jinn pre-episode I (Jinn, often considered to be more Gray than his fellow Jedi Masters), Obi-Wan before, during or after the prequels, Darth Maul or Savage Oppress, perhaps even Anakin Skywalker or my personal favorite, Asajj Ventress.
Rogue One
Considering Rogue One is set to take place shortly before A New Hope, it seems likely that this version of Galen Marek wouldn't live to see the another day. However, it allows us to revisit Darth Vader once again (perphaps played by Hayden Christensen, but as always, voiced by the great James Earl Jones) as Marek goes to face him - possibly Sidious too - in order to buy the rebels the time they need to steal the plans. But even if Galen Marek bites the dust, at the very least we would get one last epic lightsaber battle out of Darth Vader, right?
All of this is mere wishful thinking from my part, but please share your thoughts below.