I believe the best all around jungler that can work in every elo right now would be Jax, As he provides good cc, good tankiness, good burst, has easy ganks, and can smash objectives. He just works for whatever you need him to be.
Sejuani is a very strong champion, she just needs to be picked in the right situation. The stacks from her e, Permafrost, also stack off of melee champions. I tend to use her in combination with champions like Akali, Leona, and Tryndamere.
If you are looking for a pick similar to Sej, Amumu and Maokai are very good picks. They both have a lot of cc, and similar clear time, but more utility than sej imo.
As for Kayn, i would say try someone like Xin Zhao, Shyvana or Trundle. They are all very strong picks right now, if you enjoy playing fighters who are a little on the tanky side.
If you are looking for a pick similar to Sej,
As for
Hope this helps,
~moon
Bronze+: Volibear
Gold+: Warwick
Diamond+: Nunu
Carry: Shaco
However top tier exempt, Sejuani is amazing. I don't think you need to swap her from your champion pool.