Reports suggest he wants to move in tge summer. However, he himself doesn't seem to bother too much. Right now I wouldn't rule out that he stays for another season. The fact that the Madrid offer included a loan deal back to us supports the impression that remaining with us for an additional season seems to be okay for him.
Personally, I'm open to any of those solutions. With Wirtz, Paulinho, Palacios and Diaby there are four exceptional attacking/midfield talents that need playing time and Havertz would certainly affect their development if he stays but then again he's our best player by a landslide and I'm not sure we'll see a player of his quality at Leverkusen in the next one or two decades (although Wirtz and Paulinho look brillant to me), so we should try to hold on to him for as long as possible.