As opposed to the OPs 7 core idea, I'll offer the idea that as a core I'd like to have at every position, 1 player that ranks in the top 10 in the league at that position. That's six positions and realistically, a very difficult do. Therefore, if I could get:
1) A top 10 C
2) A top 10 D
3) A top 10 G
4) A top 15 W
5) A top 30 D (preferably other side)
6) A top 30 W (preferably other side)
I would then feel we have the core to contend. It would be the support cast to this core that would make the difference. In this regard, I'm quite happy seeing the Leafs collect as many draft picks as they can. The support cast can come from anywhere, the more picks you have, the greater the chance you have of developing a solid support cast: quality players throughout the line-up. It's finding those top 'core' like players that is difficult. We have a potential top 10 D in Reilly, maybe (???) Nylander at C, perhaps Marner (???) will be a top 15 W (I doubt C) but these are still to be determined. JVR, if he stays, might be a top 30, but after that, who? We still have a long way to go and I hope that management continues to draft highly skilled players increasing the chance of gaining strong 'core' players.
FWIW, I define 'top' this or that by way of a players overall abilities, not by being a power forward, a true sniper, a shut down D, etc.