Usopp is likely going to face off with Van Auger...
I already know how he's going to beat him, though. Or at least I have a very good idea.
Van Auger relies too heavily on his Marksmanship and Devil Fruit. Usopp's Observation Haki is actually superior to everyone else on the crew when it comes to sniping. That's his one redeeming quality in combat outside of his antics with his various projectiles and gimmicks.
He is probably going to shoot Van Auger from a super long distance similarly to what he did with the Sugar incident, but he needs mastery of that ability to properly use it while Van Auger is warping and shooting at him. I think the part where he can see people through walls and stuff like that also allows him to sense intentions of those people. So, it may be possible for him to both strike him from a long distance and predict where he may go.
In conclusion: I think Usopp is just going to further advance his unique type of advanced Observation Haki.
Or maybe Sogeking will come back for that fight.