I think of the two (Sig and Dembele) that Sig is the natural replacement for VdV if he leaves. Thats a big IF though, I think he's brilliant for us and as far as I understand the media coverage then Hamburg want him but the difficulty is that Rafa isn't 100% sold on leaving us and we also aren't too keen to sell. That said I think that if a big offer comes in for VdV then Levy may well pocket the funds, and while this would sadden me I can see the logic behind it.
Dembele is the Modric replacement for me. IMO replacing Modric's passing range and ability is far easier than replacing his ability running with the ball. When he is on form running at opposition players with the ball then you really see his true class, he's magic. The really top players - Messi, Ronaldo, Iniesta etc. - have this look about them when they run with the ball, they go past defenders like they're not there, and hardly seem to NEED to throw stepovers and that sort of stuff at all. And while I'm not saying Modric is on their level (yet, I think we'll see more to come from him at Madrid...) he can do this to teams. Dembele has the vision and ability to distribute like Modric, but he also looks like a good runner with the ball, as well as having more physical strength than Modric which is what makes me think he'll be partnering Sandro, Livermore or Parker in the '2' of our 4-2-3-1 that we seem to be playing. I've been wanting us to sign him all summer tbh and I'm thrilled to have him at the club. Great potential in him at 25 years old as well.
I really don't think we need more midfielders at the club now. If tomorrow we signed Butland and a 'lone striker' then I'd be over the moon with our transfer window. Don't think that'll happen but even so I'll still be reasonably happy with where we are.