Sai isn't relevant and he's appearing.
Shika is the author's pet so he's different...
Hinata is a character that needs to show growth over that TS. She can't possibly take over the Hyuuga clan in the state she is now. That's why I'm looking forward seeing her and am not worried at all. Either she'll just be passing by like in RTn (man I wish we could have got more of kittennata and Charasuke !
), or Kishi could use the opportunity to show she's finally a bit of a badass in that timeslot... Or, at worst, he could use her as a naruto-kun spewing machine and even then she's no competition for Sakura at all.
You're worrying everyone over nothing.
I've explained this in my earlier post Shikamaru has relevance because he's going to be Naruto's right hand man when Naruto becomes Hokage, out of all the Rookie 12 (excluding Team 7) he has had the most screen time, he's the only one who has had his own arc, the part where Asuma died that entire arc was centered around his growth. He beig there makes perfect sense.
Sai is still part of Team Kakashi, I know he's not had much screentime now Sasuke is back in the picture but he is still part of the team, and he's also former ANBU Ne so he could potentially to take over the Anbu squad, so yeah him being there makes perfect sense.
If this is the last part of the story it will end with Naruto becoming Hokage, so showing the people who will back him up makes sense.
Hinata showing growth and taking over the Hyuuga clan is insignificant, her being a competant leader means nothing to Naruto's storyline and him becoming Hokage, it would be a side story that has no relevance to the main plot. So makes no sense in her being there.
Am I worried it means NaruHina is canon? No because I've seen in the past how we get trolled by the Anime Studio and Kishi multiple times. However her being there does raise questions, and i'm not going to dismiss it completely, and even if it does hint at NaruHina, I'm more OK with it than I was before Neji died, she's shown some growth to make her pairing with Naruto make sense, My biggest issue would be NaruHina would also mean SasuSaku and that pairing is just wrong, that isn't a love story it's a schoolgirl crush that never matured, and would be an insut to all the growth Sakura has shown. I'd still prefer NaruSaku because that is the pairing that makes the most sense to me.