United States Championship: Rusev (c) vs. Rey Mysterio vs. Mustafa Ali vs. Samoa Joe
I'm assuming that Rusev will retain his title at the Royal Rumble against Shinsuke Nakamura. After that, one of the men who has been terrorizing the rest of the SmackDown roster, Samoa Joe, might be thrust into a program with Rusev over the U.S. Title.
WWE often tries to cram as many superstars as possible onto the 'Mania card and understandably so. Everyone wants a WrestleMania moment and every superstar deserves at least one.
Hopefully, it won't be relegated to the pre-show, but the U.S. Title isn't held in as high regard as the Intercontinental Championship.
Mustafa Ali has been treated like a big deal since he's debuted on SmackDown and he should be. He is one of the best in-ring talents and is a good role model inside and outside of WWE.
For that reason, he could be headed for a showcase match in April. If it does turn out to be a multi-man match, fewer men could match the pace of Ali except for Rey Mysterio. Mysterio still has the crowd behind him and is a threat to win any mid-card title.
Others like Jeff Hardy or Andrade could be involved as one of the four participants, but I'm going with the four men I picked.