Josh Allen had a breakout year last year with newly added receiver Stefon Diggs, but personally I'd need to see what we saw last season before I'd be willing to put him in that category. Gonna go ahead and put it out there: While he may have a unique and possibly inappropriate relationship with massage therapists, Deshaun Watson, in my opinion, should be on the list over Allen. Watson had a phenomenal season in the dumpster fire that was the Texans organization last year. The rest of your list is on-point, though. Also, while Lamar has taken Baltimore to the playoffs every year sincd he has been there, I just don't see him being able to win big games until he becomes a better thrower of the football. I see him continuing to win and put up highlight reel worthy big run plays in regular season games, only to be shut down and shut out in the postseason. I think any team would take him over Josh Allen if both were available at the same time. I also see 2021 as Justin Herbert's coming out party-- guy made some awesome plays and stats for a rookie on a terrible team.
I refrain from putting Watson on the list simply because we don't know how much playing time he gets, and even if he gets a minimal suspension, that affects his impact on the team compares to everyone else on the list this upcoming season.
It's true Diggs and Beasley (LOL Cole Beasley) helped Allen's passing a lot, and I'd love to see him take another leap forward. But, even then, the season before, Allen made John Brown look like DeAndre Hopkins in the 2019 season. He's got the ability outside of elite receivers; it just needs refinement (I think he learned a lot from that brutal loss to the Texans that year) and it's happening rapidly. With established, healthy receivers, it's a I put him over Lamar in the top 5 list.
On the other side of that coin, you're right that Lamar needs to prove himself a greater thrower of the football, which he did in 2019 with over 3000 passing yards. The trouble has been consistent receivers (his passes were on target in that Tennessee game, his receivers just decided to juggle and drop like the Ringling Bros.). I truly think he'll establish himself a better passer with Bateman and Watkins (especially if the latter remains healthy).
Yes, Justin Herbert's got a shot at an even bigger breakout season, if only the Chargers solve their ownership fiasco and get their shit together.