Alright so I am just going to go ahead and list all of my assumptions and theories regarding the future of the show, inparticular Volume 4 and certain characters... (and I will possibly be adding to this later on, just FYI)
1. This new Salem character, who is seemingly the real big bad and the person Cinder and co. serve. Now my theories for Salem are, A: she is as many people have stated the original Winter Maiden from the tale as there are similarities (I dont know about this because if she was then surely the Winter Maiden powers would have never been passed on which I assume has happened) or B: she is a corrupted/evil version of Summer rose, I only assume this one due to them showing Summers grave and then revealing Salem, however I do think it is far more likely that instead of her being Summer she is simply the person who killed Summer or has captured Summer (as Summers death isnt known %100 all sources other than songs make out like she just never returned) either way though she is quite clearly some sort of Grimm queen or something, so likely has the ability to control or at least influence them.
2. Then theres Pyrrha... poor poor Pyrrha, now her death has been confirmed as having happened on Twitter by her voice actor Jen Brown, unless thats just what they want us to think*puts on tinfoil hat*, I believe she will eventually return in some way and my theories for this are... A: She is alive and was teleported by Cinder to Salem to be used as a prisoner, or maybe to be corrupted and turned into some evil warrior for them to use, or Emerald had a change of heart and used her powers to make it appear as though Pyrrha had died at the hands of Cinder whereas in reality she hadnt, this seems really unlikely but still,(either way judging from Red Vs. Blue *cough* Church *cough* Rooster Teeth have a habit of bringing people back from the dead. Then there is B: Pyrrha will return but in Flashbacks only, this seems like a given and I would imagine these short scenes would have already been recorded for or they will just be rehash scenes from earlier episodes now I would guess these Flashbacks would be Jaunes and lastly their is my personal theory... C: Pyrrha will return as some kind of Guardian angel and/or spirit for Jaune, my reason for believing this is mainly to do with her words when unlocking Jaunes semblance "For it is in passing that we achieve immortality. Through this, we become a paragon of virtue and glory to rise above all. Infinite in distance and unbound by death, I release your soul, and by my shoulder, protect thee." and the fact that to me it seemed as though her death portrayed her soul leaving her body as she vaporised and then getting blown into the wind, so I think this could serve as a nice way for Jaune to discover his semblance with him being in some kind of trouble (life or death sort of trouble) when he goes into a sort of limbo where he meets the spirit of Pyrrha who unlocks his semblance allowing him to get out off the trouble he is in, and as well as this would allow for them to have a much needed final goodbye.
3. I feel the beginning of Volume 4 will show the fate of all the different characters, showing us where Blake went, showing Weiss in Atlas with her father and sister (WInter) as well as showing Yang and Taiyang possibly with Tai trying to convince Yang to believe in herself and the world again, and of course we will see Ruby, Jaune, Nora and Ren (Team JNNR, Journey? Team RNJR, Ranger?) and also eventually seeing Qrow, Glynda and Ironwood as well as their fates. Personally I feel this will lead the way for 4 separate story arcs one for each member of team RWBY and will coalesce at the end with team RWBY getting back together, likely Ruby's arc will clearly be going to Haven and learning more about herself, her friends and the world. Weiss's I think will be more to do with her family and heritage etc. Blakes I believe will be learning to be more open and trusting to others whereas Yangs will be her trying to get to gripes with everything. Basicly I think the next Volume will be a very character focused one revolving around their development.
4. Ozpin, I have a feeling we will eventually see Qrow (or maybe someone else) find Ozpin which will give us a flashback to Ozpins fight against Cinder so we can see that in all its glory and see what happened. Although I dont think Ozpin is dead (yet) I do believe at some point he will indeed die and Qrow will take over his position fully. Also I think, and this is purely based on one scene in the Volume 3 intro, that if Qrow does take over fully I could potentially see Winter serving as Qrows version of Glynda, so too speak, now if this did happen it obviously wouldnt happen for a long time or at least after alot of character development, and I think this mainly because of the similarities between Glynda and Winter such as them bother being serious people who despite that do care for the people close to them (this whole Winter prediction dosnt have much evidence to it, but its just an idea I like)
5. As for Remnant itself, it seems the other Kingdoms (as stated in Volume 3, Episode 12) dont know what actually happened so as far as they know right now Atlas plotted an attack against Vale and where behind everything that happened, in which case I imagine tensions will be high and Atlas (who is already seemingly seen as rather bad guys by the other kingdoms because of how the Military, Government and Schools/Academies are all one thing) will be kinda villainised by the other Kingdoms so its possible the world might soon be on the brink or war, however if my predictions regarding the next Volume being mostly character focused turn out to be accurate then I dont think this will come into play fully until Volume 5 as I would imagine Blake and Yang are still in Vale and as such dont have communications Ruby and co. will be travelling to Haven/Mistral and so wont have any access to news or anything as well, and I would guess Atlas would be trying to keep things under wraps.