Berkshire called him a super Gardiner. Better offensively but worse defensively. If that is the case, he should take over Rielly's spot on the #1 PP and free up Morgs to help with the PK.
I think that the Leafs will continue to run 2 PPs about equally. I think that they will separate Matthews and Marner again - going Tavares/Marner and Matthews/Nylander. Rielly will be on one of the PPs, and Barrie on the other (I would say that Barrie with Matthews and Rielly with Marner is probably the way things will go - I will explain why at the bottom of this post).
and a better fit for one-timers from marner since he shoots right unlike reilly and muzzin
Barrie doesn't shoot a lot from the point on the PP.
Dellow had an article in the Athletic in May 2018 called "How the Leafs’ excellent top power-play unit got even better and what your team should learn from it" where a major reason that was given for Marner's PP being much better in 2017/18 than 2016/17 is that they had subbed out RHD Zaitsev in favour of LHD Rielly. They actually have a clip showing pass after pass from Rielly to Marner that probably wouldn't have worked if the D was right handed.
On the other hand it has been talked about in an athletic article how Matthews would be more effective on the PP (assuming he stays on the left wall - which I think undoubtedly he will) if he had a RHD passing the puck to him. That is how Barrie and MacKinnon were so effective (MacKinnon - a righty - played in Matthews' exact spot). MacKinnon was second in the league in PP shots last season (behind only Ovi - who incidentally also plays in Matthews' spot and has been fed for years by RHD Carlson - and before that RHD Green). But actually a really common sequence was Barrie passing it to MacKinnon on the left wall who royal road passed it across to Rantanen on the right wall who one timed it. Regardless, in the PP videos I have watched with Barrie he seems to have a significant preference for passing on this strong side and towards the left wall. Matthews and Nylander could also experiment with switching sides from time to time (although that was a failed experiment in early 2018).
As Babs is not going to play one PP much more than the other there is no reason to stack one of the PPs.