#2 The build
Having the two men stay away from each other will not help to generate buzz this time. WWE must start on a high note. The RAW after the Elimination Chamber PPV must close with both men brawling all over the arena. This gives the fans a glimpse of what to expect.
The brawl would instantly become the talking point of the episode and help generate the buzz and excitement for the match. Lesnar can then stay away from RAW for an episode or so while Heyman hypes the match.
Roman Reigns must come out each week fighting off challengers and establishing himself as the Top Dog and as the most credible performer on the roster. This is what worked for John Cena during the US title open challenges and that helped fans turn their opinions of him to a more positive one.
The RAW before WrestleMania should end just like the RAW after Elimination Chamber, both men brawl as security and the rest of the locker room try to separate the superstars. A little blood will certainly help, but that's not something we can expect from WWE.