Kahless (clone) became an emperor in the Klingon empire during TNG. Wouldn't it have been interesting to see him during the Dominion war? Weird that we didn't, he might have something to say about the war, even if the true political power remained in the high council after his return.
Kahless is one of those things that the writers hand-waved because they didn't want to worry about it. There's nothing that interesting they could have done with a purely symbolic leader other than have him say stuff about honor now and then, so at most he would have amounted to a few throwaway lines that would have to be explained to anyone who didn't see the original episode. So they just ditched the character instead.
If they had brought him in for an episode, it probably would have been for a disguised Founder to assassinate him or something.
According to MA, he was in DS9, just as a small part and a handful of references http://en.memory-alpha.org/wiki/Kahless_(clone)
He's referred to by name in DS9, but Memory Alpha says he was played by Kevin Conway, who only appeared in one episode on TNG. It's a case of a character's importance being played up in dialogue rather than screen time.
The thing about Kahless is that even for a standalone episode the idea seemed a little odd that the Klingons would accept this clone as a moral symbol. The Klingon narrative at the time in TNG is that the Klingon high council was losing its way and needed guidance to get back on the right track, and that narrative is not compatible with the Klingons becoming enemies in DS9 season 4. Asking the writers to bind their hands with a character whose concept was kind of silly to begin with would have hurt the show. Just like if Federation ships were permanently under orders not to travel faster than warp 5. What might have been fun is if they showed some kind of Klingon parade where Kahless was sitting their waving to the crowd with some Klingon remarking "He's supposed to be our leader but that's all he seems to do nowadays." I would have rather Martok been the one to kill Gowron. One thing I didn't quite like about the way they handled Worf's character is that all of Klingon politics seemed to revolve around him when he was not in fact that important a Klingon.
Kahless' role would seem to the similar to the current Emperor of Japan, the spiritual leader of the nation.
Kor talks about him derisively in The Sword Of Kahless: KOR: The Emperor is a pretender, a clone of the original Kahless cooked up in a vat by ambitious clerics. And if you hadn't supported him, he'd be right back in that vat where he came from. Kor was revered among the Klingons, his opinions had a lot of influence.
With Phase II, it would have been even more Japanese. The Kitumba was supposed to be the equivalent of the Emperor and the Warlord would have been the Shogun. A symbolic role into a consitutionnal monarchy's the only plausible way to give a political place to Kahless considering the meritocratic aspect of the Klingon regime comes from Kahless himself. I don't think the High Council and the Chancellor would be interrested to weaken themselves by giving effective powers to the clone (especially since the clone could be challenged and killed).
Martok finally gave us a 3 dimensional Klingon that didn't just give lip service to Klingon ideals. I don't think there is much they could have done with Kahless that they didn't do with Martok.....and Martok had a lot less tricky baggage to deal with.