Lesnar as champion in 2017 could happen in my eyes. In 2016 Lesnar wrestled in 5 PPVs (Rumble, Fastlane, Mania, Summerslam and Survivor Series) and also wrestled in multiple house shows.
Same thing happened in 2015 as well.
Now, after Wrestlemania has ended, Lesnar will already have wrestled in 2. Summerslam will be the 3rd one. So Lesnar still has two more matches that the WWE can add between WM and Summerslam, which has 4 PPVs in that time period, for RAW alone. Take out some of Lesnar's house show dates and turn them into PPV matches and I think Lesnar can have a good reign. He won't be there every week, but him wrestling every couple of PPVs could work.
The problem with Lesnar in 2014 was that he disappeared from September till January. 3,5 months. If WWE can lower Lesnar's absnces to 1,5 month from 3,5, then it could work.