The impression I get from watching these games is that Ponomarev can play right now and he fits in Sullivan's system both as a winger and a center, whichever the team would play him at. I think he'd be ready right now and he will be a good fit wherever you play him in the bottom-6. That being said, I have no issues with him playing big minutes in WBS to start the year.
Broz isn't ready yet, but he's significantly further along than I think anyone anticipated him to be at. The fact that he has bulked up to over 200 lbs is awesome, I think he is significantly more likely to be able to stick at center at 5'11" and 202 lbs than if he was still at the 165 lbs he was drafted at. His skillset has all of the makings of a good 2-way 3C, but I don't know that his upside is much higher than that. That being said, I wouldn't be surprised if he'd mesh extremely well with Crosby either. He's a jack of all trades guy, but so was Rust and I think Broz is noticeably more skilled than Rust.
I think McGroarty is NHL ready right now, but I am unsure of whether rushing him to the NHL is the best idea. He has legit NHL size, he's smart as hell, he has good hands and he has a sick shot. That being said, I don't know that I'd want him playing bottom-6 minutes with this team over dominating in the AHL. They could do the same thing they did with Guentzel by letting him dominate the AHL and only call him up to play in their top-6. Doing that could also let you have Ponomarev in the bottom-6 to start the year as well (O'Connor as LW1 and Ponomarev as LW3 or LW4), I think there's a pretty good argument for that.
Those three are the big standouts to me from what I've seen in the two games, and that seems to be a common opinion I can see on twitter as well. If you held a gun to my head right now and made me pick, I think I'd start the year with McGroarty in the AHL and Ponomarev in the bottom-6. That's not a criticism of McGroarty's ability, but I think that's a better development path for this team to take with those two.
I noticed this about Koivunen's skating as well. His IQ, playmaking and hands are all great, so I think he will make it to the NHL, but I think his skating will be a real detriment towards him. He really needs to improve that, if he does I think he's a great bet to be a middle-6 playmaker like Jussi Jokinen.