I think Tower of God should end at the actual top of the tower with the stars (whether or not Rachel is there to see them too, who knows), and Jahad is definitely not going to be defeated on top of the tower. How long the story continues beyond Jahad's defeat would depend on how many floors are left and why exactly did the original part of Jahad and the great warriors stop the climb.
I think Jahad may be the main villain/antagonist of the story in terms of relevance during most of the story, but the final villain could be someone else who manipulated him and the rest into becoming the rulers of the available floors. Admins in general, with Headon top among them, seem very suspicious to me. Perhaps there's some kind of "admin of all the admins" figure that might have been behind Jahad and the 10GW decission of quitting their climb and the general misery of the tower inhabitants who are not poweful enough to climb it
