Regarding the outrage of Krug being on the "bottom pairing"...Vatanen, for instance, is on the Ducks "bottom pairing", but still managed to have MORE average TOI than Krug or "top pairing" Carlo did last season. Shattenkirk has been getting his 20 mins from the bottom pairing for a few years now. I think everyone on these boards understands what Krug brings and loves Krug and when you have a deep D group your deployments are going to be different than when you have a primo pairing playing 25+ mins a night and you hide your bottom pairing.
I think you have three options with the D... I'm going to use "MKC" to abbreviate that bottom pair RD of one of McQuaid/Kevan/Colin.
Option 1:
Chara McAvoy
Krug Carlo
XXX MKC
In this scenario, like they did with Hamilton, you will probably see the Bruins give the Chara pairing less difficult deployments than he got the last couple years to put McAvoy in some more offense-friendly situations. This means that either Krug/Carlo or the 3rd pairing gets some harder minutes. Count me as firmly against this idea. I really like Carlo playing with Chara for Carlo's development. I think the Chara/Carlo is an effective shutdown pairing and is giving Carlo the experience to handle the matchups he will get when Chara moves on. I also think that playing Carlo with Krug (who you want to give offense-friendly deployments to) is a waste of Carlo.
But, if you do go this route, you want that final LD to be someone who can play PK and makes that pairing one that doesn't need sheltering because you can't afford to shelter it if you want to give the McAvoy pairing AND the Krug pairing offense-friendly matchups.
Option 2:
Chara-Carlo
Krug-McAvoy
XXX-MKC
Like option 1, this lineup is best if you don't have to shelter your bottom pairing because you really want to give that 2nd pairing all the offensive chances. This is a very very interesting idea, but still really requires an addition to the bottom pairing to make it one you don't have to shelter because Krug/McAvoy will get all the fun minutes and anytime you have to protect the bottom pairing, you are stabbing Krug/McAvoy in the back. Watching Ellis/Josi makes me wonder if this could work, but I also wonder if there are enough pucks to go around for Krug and McAvoy if they are on the same pairing.
Option 3:
Chara/Carlo
XXX/McAvoy
Krug/MKC
You can swap the order of the McAvoy and Krug pairing if that floats your boat. Heck, you can swap the order of any pairings in this setup. You have a shutdown Chara/Carlo pairing and then two balanced pairings, each with a guy that can be a threat in the offensive zone. Notably with the Pens moving Kessel to the Bonino line last year, but in general, there has been a (smart) movement towards spreading out your offensive talent across your forward lines to be a threat at all times. I think the natural evolution of this idea is to do the same on the backend. The XXX you would need to compliment McAvoy also happens to be the type of D that can help take the load off Chara.. a LD that can play on the PK and you don't have to hide from harder matchups. Someone like Brodin, DeHaan, etc.