1. The Black Order Will Show Up In The Post-Credits
Where infinity stones go, Thanos, or his Black Order at least, are sure to follow. Since collecting all six stones is The Mad Titan's entire motivation going into Infinity War, the soul stone is certain to be on his radar.
Thanos himself will not turn up, instead he will send someone to do his work for him. We have seen him do this all through the MCU, using puppets like Loki, Ronan, and Nebula rather than getting his own hands dirty. This time he will likely send a member of The Black Order, however, whether we actually see them or they are just teased (as in the Ragnarok mid-credit scene) remains to be seen.
As discussed previously, Black Panther will largely be a stand alone movie, yet there still needs to be a reason to bring T'Challa and his Wakandan army into Infinity War. An invasion by Thanos' forces would certainly bridge this gap with a scene pulled straight from 2013's Infinity comic arc, in which Thanos sends The Black Dwarf into Wakanda.
In the same way that Ragnarok teased The Black Order hunting Loki and the Tesseract, this would leave audiences on an incredible cliff hanger before Infinity War hits later in the year.