Even though I never have been a fan of fast travelling in games and while it was not much needed in TW1 and 2 (in 2 I always got 2 points on roll distance and spammed space to fast travel 
) I'm afraid that if the game is huge it will become a necessity, if I judge TW3 while having the prequels in my head then the answer would be that fast travel won't be needed at all, but it will be a very different game and open world, and while I'm sure it will be a very beautiful world to explore sometimes it might get frustrating to go back and forth, so I think that fast travel in the witcher 3 is a must, even if I don't like open map click and go thing, for me Morrowind's fast travel was good enough, with Silt Striders (like caravans in skyrim) and the spells Almsivi and Divine Intervention (fast teleport to temples and forts), now I know that whatever they do it has to stick with the lore so Geralt casting a spell won't work, also the stones are out of lore for me(only read 4 of the books so far so I might be wrong) waypoints like Diablo I don't know either, so maybe the open map click and go way is the only way it can work, in any case I hope that in whichever way they implement it, it won't become an abusive mechanic, just my personal opinion
) I'm afraid that if the game is huge it will become a necessity, if I judge TW3 while having the prequels in my head then the answer would be that fast travel won't be needed at all, but it will be a very different game and open world, and while I'm sure it will be a very beautiful world to explore sometimes it might get frustrating to go back and forth, so I think that fast travel in the witcher 3 is a must, even if I don't like open map click and go thing, for me Morrowind's fast travel was good enough, with Silt Striders (like caravans in skyrim) and the spells Almsivi and Divine Intervention (fast teleport to temples and forts), now I know that whatever they do it has to stick with the lore so Geralt casting a spell won't work, also the stones are out of lore for me(only read 4 of the books so far so I might be wrong) waypoints like Diablo I don't know either, so maybe the open map click and go way is the only way it can work, in any case I hope that in whichever way they implement it, it won't become an abusive mechanic, just my personal opinion