Some good kids coming through like Weigl and Hoffman so hopefully we can keep at least Reus for another couple seasons while they mature. He seems to love the club though, so if we qualify for champions league this season he might commit for longer. I love Hummels but he wouldn't be as big a blow. We've already got Sokratis, Subotic and young Ginter who is the next Hummels, has already made the national squad too (I think he was in the WC winning 11 too?). Ginter is a beast at full back but also has the pace to play RB opposite Schmelzer.
I don't think Reus is make or break for BVB anymore though. He's an amazing player but I don't think we'll be so lost without him with Kagawa, Mithikaryan Castro, Sahin and Gundogan already there though granted that probably only Mithi and Gundogan could play Reus's attacking mid/forward role. Blaszczykowski is on loan to Fiorentina so we would have him back too. If we make champions league then we may have to sell someone anyway to sure up the strikers. We've only got Aubameyang who is a gun but the only real backup is Ramos (not the good one). That won't be good enough to challenge all of Bundesliga, dfb pokal and CL.
And there's my BVB/Bundesliga 2cents