Not just in this world but in different dimensions as well, maybe? That gives opportunity to introduce new and more powerful monsters. A new world overflowing with monsters and welcoming monster slayers.
Adding new types of monsters is definitely interesting, but the way I see it is that it does not have to be a different world setting. For example, maybe the short effect of the second conjunction of the spheres in W3 resulted in the invasion of new threats from different dimensions, which requires the unique set of skills of witchers in order to face the new dangers.
Also character creations, because we all want to be our own "witcher" with our own storyline honestly. Have the option to be a classic witcher with mutations, or a new kind of witcher like Ciri. Or be a Mage, or hunter.
Considering the three endings of W3 may be helpful in this case. For instance, the ending where Ciri becomes a witcheress can be used to have Ciri as a second playable character, along with Geralt, that have more than one skill path that can be tweaked towards more mutation skills or more magic skills etc... This dual character mode will allow players to have a versatile gaming experience, especially in combat and interactions, that can provide a unique experience to each playthrough, such as the story of Chris Redfield and Piers Nivans in Resident Evil 6. This can also apply in the ending where Ciri becomes a queen as Geralt and also either Yen or Triss, but it will be difficult in case we had 'it takes three to tango', will become her trusted companions, but I doubt becoming a queen would involve learning magical skills or acquiring mutagens, so maybe Ciri would learn such skills in secret or maybe it has to be something else entirely. Similarly, in the ending where Geralt hunts the last crone, Ciri's skill set would take another direction as she will be lost between different realms, after she ecountered the white frost, so maybe she would become something else in these realms until she can be reunited again with Geralt.
Also have the freedom to choose schools, like wolf or viper, or mage, or learn something from all of them.
This can be perfect for the continuing the line of quests of finding the different schools' gear in W3. So Geralt will continue his journey to find the special mutagens of the different schools which will unlock multiple skill paths that can be customized by choosing different skills from different schools that can create new sorts of combos.
Have the journey start out as a young kid, kind of like the trial of the grass, then progress to adulthood. Depending on what school you choose, have the old characters be your mentor. Like an older Geralt would be your mentor for the wolf school like Vesemir was in W3. Have him accompany you early in the game, also have Yen or Triss in there (depending on who you chose in your game) to teach you magic if you choose no mutations.
I think it would be quite interesting to experience the journey of becoming a witcher. I believe it would be quite fitting to have this phase also focused on Ciri as Geralt will be her mentor and we can possibly experience more flashbacks of Ciri while she was at Kaer Morhen, and that maybe she secretly passed the trial of the grass with only Geralt's knowing of this. Then in the present setting of the game, Geralt continues to teach her how to acquire new skills, and Yen or Triss walking Ciri through honing her magical skills. But again I do not how this training of either Yen or Triss will play out in case we had 'it takes three to tango'
Have the ability to create and choose your won weapon. 2 swords no longer needed, any silver and steel weapon will do.
I just wish that we would get a more complex crafting system. So instead of crafting a full sword from a single diagram, maybe we should be able to craft each part of the sword or weapon and combine different parts together to get unique effects. So for instance, the blade, guard, hilt, and pommel parts of a sword can be separately crafted and matched with a certain diagram or make a hybrid one.
It would also be awesome to have endless hunts or siege battles or fortress defense. The defense battle at Kaer Morhen was really fun in W3. Like defend Ciris castle in the new game (depends on the ending you chose in your game), then Include battle strategies in the defense/offense of it. Upgrade your fortress, say your school or something as you progress in your game.
Indeed. For example, the system of Shadow of War offered a quite enjoyable fortress sieges. But I felt that it became dull after completing the main story of Shadow of War, so maybe, if applied in a witcher release, it needs something more. But yes, it should be dependent on the ending we reach in W3. In fact, making this fortress siege system dependent on each of the three endings can offer more varied system that takes ages to get bored from. For example, in the ending where Geralt hunts the last crone, this can allow us to capture and defend monster dens instead of fortresses as Geralt would find allies among monsters, similar to Talion, after defeating the ladies of the wood. In the ending where Ciri becomes a witcheress, we can rebuild other schools' fortresses, including Kaer Morhen.
By the end of W3 B&W, you had all these things but its a little boring when theres nothing left to do but revisit everything and kill weaker opponents. It would be nice even after you beaten the game to be able to continue to go on quests hunting strong monsters or hunt down criminals, trying to locate them in your world. Have tournaments and battle other witcher schools or something.
This could be an online thing. Build your own reputable school/guild of sorts, have the ability to customize/upgrade it. Have your own party/members/army, then defend it or conquer another players online. Have quests in another players world, like join up to raid monster dens/dungeons.
Having this feature would make the game forever playable, in an ongoing term. You would get to enjoy your endgame gears and weapons longer.
I completely agree with this.