A partner for De Jong has to be a player who's able to create offensive opportunities when left open (in case De Jong being manmarked) or willing to make runs towards the box to create more space for De Jong to operate. I think Fred might be good enough, he can make runs or dribbles, technically okay, and probably more comfortable playing a bit higher up where he can take more risks in his actions (De Jong will take a lot of risks playing deep, but that's not a playing style suited for just anyone).
If Eriksen signs with Man Utd, I would be interested in watching a midfield of Eriksen - Van de Beek - De Jong. Maybe I'm too much of an idealist, but that midfield would have so much technical ability, passing ability, creativity, scoring ability, IQ, offensive awareness and selflessness. Defensively I'd count on De Jong tracking back.