Around 16 PPG, so long as he's playing 33-34 minutes. If he's playing 25-28 then closer to 14 ppg.
I don't think Wall will crack 20. Beal definitely won't. Both are very willing passers and the Wizards have too much of a varied attack. No team can play Wall or Beal one on one, and no defense can handle Gortat or Nene down low without collapsing. Pierce will be the only guy you can leave open (notice Ariza's career year).
While I agree that Pierce will get open looks, I find your second paragraph confusing.
The Wizards weren't an offensive juggernaut last year, they were a little below average. Given they're bringing back largely the same core I don't see them improving much more than a slight bump as Wall/Beal improve.
Which part confused you? Each statement is pretty straight-forward and self-explanatory.
I did not call them an offensive juggernaut, I said they had a varied attack. One second they are beating people on the perimeter with speed (which is why you have to help whenever Wall or Beal drives) and the next second they're lobbing it into the paint and playing an inside-out approach with plays that include significant big to big passing. Then one the ugliest benches in the league (outside of Miller) comes in and screws it all up.
This style results in lots of ball movement and those touches keep guys in rhythm, which we know is huge for Pierce. They don't have a single selfish player causing the ball to stick. They were tied for 7th in APG despite being 18th in pace. If they had a bench anywhere close to the Spurs they'd be right behind them.
Wall - 8.8 apg
Beal - 3.3 apg
Ariza - 2.5 apg
Nene - 2.9 apg
Gortat - 1.7 apg
Again, that's with the 18th quickest pace. And then Miller off the bench with 3.5 in 15 minutes (same as Wall per-36). No other teams have starting big men that average almost 5 assists. For fun, how many teams in history can you name whose PG has 9 APG and two other starters have at least 3? Tons of high pace teams in high pace eras to choose from.
I agree with you that there's no reason to think their offense will improve any, but I never made that claim either. Pierce should fit in fine, he had the same number of assists as Ariza while playing 20% fewer minutes.