After reading a bit, it seems like the 2 major questions here are:
1. Is there any sort of option to buy for Dortmund?
2. Is this part of an eventual move for a Dortmund player?
If the answer to both is no, then it seems odd we'd loan Januzaj out to Borussia Dortmund instead of say Borussia MGB or Mainz or Hoffenheim or somewhere he'd probably start. Right now, Dortmund play:
Reus-Aubameyang-Mkhitaryian
---------Kagawa---------------
as their front 4, with Hoffman and Ramos getting sporadic minutes backing them up as nobody has been hurt yet. Kuba left today and the other attacking players (apart from them moving Gundogan or Castro up into the 10 role) are Leitner and Grosskeutz, who appears to be joining Galatasaray. So, really just Hoffman, Ramos and maybe Leitner, who was rumoured to be going off on loan.
Considering the amount of Europa League games (and DFB Pokal games) Dortmund are likely to play and that Tuchel is said to not really rate Ramos, this isn't as crazy a loan as it first seems. They really are a bit short of attacking cover. They'll likely hit at least 50 games and that's 200 attacking starts. No reason that with injuries, resting and rotation Januzaj can't beat out Hoffman and/or Ramos to be 1st or 2nd in line to backup at least that front 3 and get 25-30 starts for an excellent side under a good manager.