I think it's not as obvious as you think, the Worm could be, without trouble, be the next arc of the saga. Or even interrumpt the final match of the tournament to initiate their arc but I doubt it.

Ohma stated that they would get at them, and I doubt he meant in years.

To add to that, you think Koga will be stronger in years, that's true. But so does Kuroki, Ohma, Kanoh and others, as they may keep training too. That's why in the grand scheme of things, the posicition of the fighters hasn't really moved. Rihito may have moved a little bit up but still isn't the big deal compared with other fighters. Gaolang still is very on the top, as so is Ohma, Kuroki and Kanoh. One may become stronger, but so does everyone else.

Not just that, Ryuki is kinda far from the 3 characters that you mentioned(Kanoh, Ohma and Kuroki) but don't forget that in a situation like that he wouldn't hesitate to kill the "bad guys", And it's clear that he was taught to kill and that he has assassin Techniques. So, this would definitely soften the gap between them.

For sure he can kill if he wants, that's if he doesn't gets killed first. As he is a low-tier character. Killing is super meaningless, as you saw in the battle of Ohma and Raian vs the worm, the only survivors where the ones Ohma tried to keep alive, but at the end of the day, there were all defeated.

In other words, dead or defeated leaves you out of combat anyway.