Nicholl hasn't really found his groove yet like O'Reilly has, he's mostly been on the 3rd and 4th lines on grinding duty. It's really hard to say what his role will be next year because there are a huge number of players who are going to be overagers on London (Julian?, Gazisov, Edward, McCue, Lawrence, George, Sim) and there's potential that Cowan sticks with the Leafs.
That pretty much leaves Barkey, Halttunen and SOR from last years team from the top 6 (Halttunen mostly played 3rd line too but was 1st PP) and Nicholl and Brzuwesticz that were on the team before for forwards (though Henry is probably is a d-man this season) and then there's a couple 17yo who might make an impact but it probably is going to solely rely on what the Knights do trade wise. It's also pretty likely that Gazisov comes back and Julian probably will too, If Nurmi comes over from overseas he'll probably also be in the top 6. Sim could also come back and get promoted to the top 6.
Nicholl could potentially take Gazisov or Sim's place in the top 6 if he performs well but it's more likely that the Knights bring back some players and trade for some players and he gets relegated to the 3rd or 4th line again. If he gets to play with Sim and another decent player he could look a lot better though.
I think there's probably still 0% chance that Nicholl and SOR play together though as I expect the top line to be Barkey - SOR - Halttunen if Cowan doesn't come back, and if he does it'll probably either be Halttunen or SOR getting bumped to the 2nd line.
We just don't really know right now since camps haven't finished and teams are still making decisions about where their prospects are going.
As for Nicholl himself he's a great energy player with a bit of skill, he has decent all round skills but nothing particularly that stands out, he's willing to work for the puck and go hard in the corners and is a pretty good forechecker because of it but he doesn't have the hands, shot, or vision to complement his other traits. I think his vision is probably his best offensive trait and he can potentially find and pounce on a puck with a quick release to score or make a nice pass across the ice to someone who's open but he's really only shown flashes of that potential. If he puts it all together he probably ends up as a 3rd or 4th liner with some skill who can occasionally mesh well and isn't a detriment to his team. His defense is generally pretty solid, it's mostly offensively that he can leave you frustrated.