So I've put a lot of thought into this cause, as a Rockets fan, it would be tough to see a bad lineup put out there for the Memorial cup after so many years of success.
One of the biggest thing to realize is that this team (barring something very surprising) will return every single forward from this season. Mattson and Topping will be two of the leagues premier 20 year olds. Both were around a point per game this year and I wouldn't be surprised if they were to push 90-100 points next year. Nolan Foote had 36 goals and will be 19 next year. Barring injury and potential invite to team Canada's WJC camp he could realistically hit 50 next year. Obviously, beyond that it gets interesting. I think the first couple weeks of next season will be very important to determining how much Hamilton will have to deal. This year the major downfall of this team was the forwards who were unable to improve upon their previous season. Kindree, Gardiner, Bruggen-Cate, Farren, Brennan, and Cowell all had either very little improvement or a decrease in scoring from the previous season. Erik Gardiner struggled and had to retire early due to injury. Cowell struggled, ended up traded, and left the WHL all together. Zablocki and Bowen did not pan out up front as 20 year olds. So essentially what I'm trying to say is that this can't happen again. Bruggen-Cate, Brennan, Farren and less so Kindree NEED to show signs of improvement early or there will need to be changes. If the next crop of Liwiski, Swetlikoff, Crosbie, and Ernst (among others) can have a natural upward trajectory that will go a long way for this team. Another big question mark is Ethan Bowen, who had a strong 16 year old season in Jr A but is committed to North Dakota. Chase Dafoe is another player on the college trajectory but remains on the Rockets protected list. With Wong already certainly taking up a roster spot as a 16 year old its hard to see them taking on many more 16/17's but they look to be great players.
Meanwhile on defence, aside from Korczak and Thomson there is a bit to be desired. They graduate their three 20 year olds from the back end in Gally, Higson, and Barberis. All that remains beyond that are Steffler and Augustine who aren't about to hit the scoresheet often but do solid work defensively. There will have to be a trade or two to bring in 19/20 year olds on defence because you can't rely on 16 and 17 year old rookies too much on a Memorial cup team. I look at guys like Jett Woo, Calen Addison, possibly even Ty Smith who are all on teams who could be trending downward next season and looking to sell.
It will be interesting to see if they stick with 18 y/o Roman Basran in net. Porter will be 19 but has been inconsistent and I'm not sure is quite what they will be looking for as a team with their eyes on the Memorial Cup. I look at the Hofer trade from this season that netted Swift Current 6 draft picks including 2 firsts. While I don't think Basran has quite that much value, he could be a key piece in a deal for a 19 or 20 y/o goaltender.
They will likely have three players who will go in the first 2 rounds of the NHL draft this summer (Foote, Korczak, Thomson) so that is a pretty darn good foundation for a contender. They also have the 5th overall selection from this upcoming WHL draft to work with. I have a hard time believing Hamilton will deal that pick just based on his history and how he likes to build teams, but it's in the back pocket for now.
TL;DR: Mattson, Topping will be significant upgrades compared to 20 year olds from this season. Young forwards showed spots of brilliance this season and will need to continue to improve into next season. Korczak and Thomson will be elite defencemen in the WHL, but they could use another good piece on defence. Question marks in goaltending could lead to the acquisition of a veteran goaltender.