Yeeaaaah... I just can't see Brock retaining the championship at Wrestlemania. It'd be an almost win-win scenario if he did since it would mean they wouldn't be wasting the rub of ending Brock's reign of dominance on Roman (I'm not as down on him beating Brock as much as others are, but I've never quite seen a babyface this cold be appointed such a big push), and by proxy it means the midcard titles will get spotlighted more by getting to headline a few PPVs as well, but for a plethora of obvious reasons that I can't be arsed typing out, I don't see it happening. Reigns is beating Lesnar, I'd bet my life on it.
That said, if Brock did retain, they could easily have Bryan or Cena headline a few PPVs and carry things for awhile in-between Lesnar's rare appearances and title defenses (that's assuming Bryan even wins the Intercontinental Title, which I don't take as being as much of a given as others do.) According to the European tour that's been booked for after Mania, Cena vs Rusev for the United States Championship is headlining the A-Crew's side of things, so the big third and final rubber match between them could easily headline the Extreme Rules PPV in April. According to the rating breakdowns for Raw these past couple of weeks, it seems the Cena/Rusev rivalry has captured the people's interest more than any other feud has anyway.