I hear what people are saying, but I'd like to be a bit more vague - if I may...
We need a stronger spine - at CB, DM, ACM & a decent CF who won't make us shit our pants if Lukaku is injured or when he moves on.
We have several decent wingers on both sides when they are in form and good enough full-backs (for Jan window at least) but a solid spine would would make us a good top 7 side.
On top of that - I've said it before and I'll say it again - we could do with signing some LEADERS on the pitch. Top sides have more than one captain/leader/talker/organiser in the first 11 and we are desperately in need of this. Although, it seems a lot of players are these days, I think standing up for what you believe in, even if it goes against the manager, is slowly being coached out of young players these days.
Above all else, I think two positions are key and should be priority - replacement for Barry and replacement/support for Lukaku.
If we sort those two in Jan I'll be over the fucking super-moon!