Imo, Shishiba would be cool. Tho I don't mind Shin doing it but a few more chapters down the line, he isn't strong enough yet imo
Another option I could see (if Gaku dies soon) is Amane's dad. He's also abnormally strong and we've only been teased with his strength
Yeah I was thinking Shishiba since he hasnt gotten a fight yet. It seems like the author is saving him for something. My personal opinion is that Shishiba will be the key to defeating Yotsumura (Amane's dad).
Idk I'd really hate it if Shin was the one who defeats Gaku. Personally I think Gaku dies the arc after this one or the one after that. Thats when I believe Slur gets defeated. So Shin getting strong enough by then, no matter how much plot armour and also nerfs Gaku gets.....would be lame. The mangaka has been consitent with Powerlevels in this manga, and Shin just isnt strong enough yet for Gaku tier. Gaku seems Order lvl or just a bit below it.
As for Amane's dad, highly doubt he'd be helping fight Slur since hes beefing the Jaa. However he may switch sides if he is possibly defeated by Shishiba (I really want Shishiba to beat him lol).
If Shishiba has a good fight against Yotsamura and he wins. Then I want Nagumo to defeat Gaku, maybe even Hyou would be interesting. Lets not forget Kanagiri (the traitor whos in the Order) also needs to be defeated. Would be nice if Takamura takes care of him. If not....then another order member.