Piazon is just awful for Frankfurt and really doesn't look good enough to be a Bundesliga player. No work rate, doesn't win a 1on1, no strength and no threat in the attack. One wonder free kick goal but that was pretty much it for him. I don't think he will have a future at Chelsea much like most of the other Bundesliga loaness.
Hazard is still your best bet but he is also not even first choice at Gladbach and is mostly used in the EL matches when the other players get a rest. I see the most potential in him though it's questionable that he will be anything more than a squad player for you in the near future. A few more years at a second tier club would benefit him most imo.