Yes, Nika will definitely defeat Saturn in my eyes.
Keep in mind that this is my perception of the story and I was the first one to say that Luffy would take down Kizaru very quickly and he may even one shot him with a G5 attack comparable to Bajrang gun. We still got a bit more than a one shot but we got the one shot. It didn't of course take Kizaru down permanently.
I'm not 100% accurate but I knew that the fight would not be long and that a named G5 attack comparable to Bajrang gun could put Kizaru down. I just didn't know it would be temporary because I did not know about Saturn's real strength.
Why do I think that Nika will defeat Saturn?
- Saturn came to Egghead because Kizaru failed his mission, to take over Kizaru's mission
- Saturn has already proven to be stronger than Kizaru
- Saturn is really posing a threat to Luffy's friends, he eye diffed them and he is about to kill them, Kizaru did not even get close as Luffy was in the way
- Luffy/Nika gets really angry when the opponent is really being a threat his friends, Kizaru was only trying
- The flashback where Kuma tells Bonney that Nika will liberate him
- The flashback where Kuma's dad tells Kuma that Nika will come to him one day
- Nika will defeat Saturn and liberate both Bonney and Kuma, it's so blatantly obvious
- That will shock the entire world
- Nika will be displayed in the newspaper, pictures and the name of "warrior of liberation sun god Nika is back"
- Nika liberates Vegapunk and defeats an elder
- The Egghead incident is the fall of the elder Saturn and marines failing in their mission of killing Vegapunk
- Luffy will get a huge bounty increase that will be at least 4.4b (will surpass Shanks')
- Zoro will get also a huge bounty increase for defeating Kizaru (3b bounty or higher)
- Luffy will meet Shanks in Elbaf and give him the SH back (Shanks will come back to Elbaf)