If you want a safer assassin pick i'd go for Kha'zix over Rengar right now because Rengar only work in certain team comps that only have one tank, if there's a tank support and a tank top lane don't play Rengar. But if there's a assassin mid, a assassin jungle, and a support that isn't tanky i'd definitely play him. This is mainly because bruiser Rengar isn't to good at least in my opinion.
Sejuani is really strong, and has been since her rework. I picked her up when ip turned to blue essence, and have gotten 5-6 s rank games on her. It mostly depends on your play style though, if you enjoy playing tanks, i would recommend playing Sejuani, Amumu, Gragas, or Zac are really good picks right now. If you enjoy playing more burst champions, i would recommend Jax, Kha'Zix, Rengar, Evelynn, or Nocturne.
Kayn is superstrong now, something like zed, there to 1shot the ADC if you're going for assassin. As far as Sejuani goes, i dont really like her after all that nerfes, but she's okay.
Sejuani is the current meta pick, because of the update and playmaking, but the runes with precision and domination are quite strong and with lethality, Kayn would fall amongst the ones I've mentioned. Sejuani will always be a tank option but I think it's heading towards assassins and bruiser type champs now. Obviously not saying that the tank junglers will not be picked. I wonder if champions like vi and shyv would be making a comeback? I've already played against way too many Vi junglers after the runes update. x_x
Hope this helps,
~moon