Defense, Goaltending, Coaching. Its going to be insanely hard for Ken Holland to fix this team, not just because of the cap crunch - but he also inherited a team with no depth, so there's no spare assets to trade. He needs to simultaneously shed cap while brining in top talent, depth talent and retaining (and hitting on) draft picks. He also needs to do this all under the ticking clock of McDavid being in his prime and not demanding a trade.
I still say and truly believe he's the best GM in the league, and will get it done in EDM if given the time and support --- but holy crap is the deck stacked against him. Even so, looking at signings like Koekoek, makes me hopeful that he can scout/snipe other teams' undervalued bottom pair D and pick them up for next to nothing and build some semblance of an NHL caliber defense. Hopefully they can get a better coach. If Columbus fires Torts for any reason, EDM should jump on him and cut him a blank check. Torts' defensive system and individual player accountability is EXACTLY what EDM needs to fix their system. We've all seen what Torts can do with a mediocre team w/o top talent up front - imagine what he could do with the current Oilers roster.
Fixing the goalie situation may not be possible for another year or two based on how the goalie market has been - but if Holland can fix the D I think a lot of the other problems will self correct.