Ginter replaces hummles and they buy in another young centreback (tah, sule, rudiger - there is plenty of young German talent there... or they can play Bender at the back)
Weigel replaces Gundogan and again they can go out and get young talent (Goretzka, Tielemens etc)
Along with Passlack and Pulisic they already have impressive young talent (weigel, Ginter and now Dembele) - they look set to remain a force for the forseable future (especially if they can keep Reus and Aubamyang)
Plus they will presumably offload Immobile this summer as well for a few extra million to invest in the squad
Ginter won't replace Hummels, because he is too weak defensively. Right now his future seems to be as a squad player for the RB/RWB/C(D)M positions, as well as 4th to 5th choice CB behind Hummels(replacement), Bender, Sokratis and Piszczek who played as sort of a RCB when Tuchel used a back 3.
Tah probably won't be available. Rüdiger is actually a former Dortmund player, but afaik the media hasn't mentioned him at all. Süle might be a candidate, but I don't think he was mentioned by a serious paper. Toprak, Dragovic and believe it or not Varane are supposedly at the top of Dortmund's list. Bender wouldn't be a bad option imho if the club sign a very talented younger CB who can challenge him and Sokratis soon.
Gündogan was apparently gone already, but after his injury a long term extension is supposedly back on the table.
Immobile has already been sold to Sevilla for around €14m.
I don't think there is any chance of getting Goretzka, because of the Schalke rivalry. According to Kicker Dortmund are looking for a combative CM, so it might not be Tielemans.
Weigl might be suitable as a Gündogan replacement, but then that would only shift the vacancy to his role. Mkhitaryan could actually play as a CM, especially if Dembele hits the ground running.
I could also see Götze as a Gündogan replacement, but this transfer doesn't seem as likely as it used to and I have a hard time imagining Bayern letting him go for a fair sum, after Dortmund stood firm with the Hummels fee.
Tactically Tuchel has been very flexible this season, so it's not that easy to predict how the main setup will be next season. Hummels always had a great deal of freedom because he is such an amazing playmaker from the back. I imagine his replacement won't nearly be as effective and thus play a more traditional and rigid role which in turn should free up parts of the midfield to take a more active role in the build up.