... hmm.
I'd say that I love Kageyama mostly because he is the perfect character for the story, as the middle-man. The Yes-Man. The grunt that fights the heroes.
But if Yagu came back, their new enemy would have to be another rider, while Yagu with a renewed sense of JUSTICE would Ally to Tendo, and Tendo alone. Seeing him as a truly remarkable man, one whose' "Kanzen Chouwa" between the other riders is an amazing thing to have.
Kagami would prolly be happy as crap. But, at the point of Yagu's arrival, He would most likely be siding with Scorpio-Worm, protecting him from TheBee and Kabuto's double team.
Eventually, a new kind of worm would appear, and a new weapon would arise. The Hyper Sword.
Everyone has no problem lending Tendo their zecter, as they all trust him, which is why Yagu would be good to replace Kageyama, who wouldn't trust anyone minus Zect (AHAHHAHAHAHA)