I'm calling it now...
Not a spoiler, just a prediction...
The guy coming after Reddington is Lizzy's father, essentially making Red Obi Wan. he took her from this very bad man, was probably in love with her mother, or is maybe Lizzy's uncle(which was my earliest guess), and the most evil man in the world finally found out he had a daughter and wants to make Red pay.
Spoilers for last night's episode, The Kingmaker.
I've really been enjoying this show, Spader is great in his shifting morality and loyalties, but above all it always seems clear he has a profound love for Lizzy. He never lies to her, even when she asked him if he killed her adoptive father he admits to it immediately, even acknowledging that the man was going to tell her something he shouldn't have, but that he was a friend for most of his life and he ended his suffering. When she asked Red if he was her father he said no, so I believe him. Now that's some serious shades of gray; between this and Hannibal, NBC is really shaking off the stink of the last few years.
Now that I think about it, the only flaw in this logic is that if he stole baby Lizzy away from an evil man and sent her to safety with a friend, that should be exactly who he suspects of tearing down his empire and coming after him, unless he had good reason to believe the man dead, which is what I expect, for now.
You may now return to your regularly scheduled programming.