Vs A/4th Raikage - I'm not sure Kakashi would be able to take this one. A's speed is ridiculous, especially upon using raiton armor. While I assume there's a possibility of a similar scenario like what happened when Sasuke fought him, where Kakashi is given an opportunity to strike him with a raikiri, but while Raikiri is a more powerful form of chidori, I'm not sure it'd penetrate enough to kill A.
Bunshin feints aren't likely to work. A raiton kage bunshin probably wouldn't even effect A, and even if Kakashi just uses a regular one, A's reflexes are fast enough to react to any "sneak attack" Kakashi performs. While I believe kamui can be an instant kill if Kakashi wants it to be, that won't really matter as he won't even be able to see Raikage long enough to even activate his MS.
I think Kakashi would give A some difficulty, but A would take it.
Vs Onoki - Kakashi is a really bad matchup for Onoki. Kakashi's bunshin feats are 2nd to none in this manga, and all it would take is Onoki using dust release just once for Kakashi to fool him with a bunshin and engage in to a taijutsu altercation, which given Onoki's old age and back issues, he's likely not very capable of defending himself in, not against elite ninjas, anyway, or even end his life with raikiri.
Kakashi's strategy of figuring out his opponent with bunshins before engaging in battles is the perfect counter for Onoki, as given what dust release does, I'm not sure he'd even notice the fact that it's a kage bunshin after using it, which wouldn't really matter anyway, as Kakashi's speed blitzing feats are more than sufficient to get Onoki.
Just a really bad matchup for Onoki, Kakashi would likely take this.
Vs Mei - I find this one interesting. A battle between two ninjutsu masters. The relatively slow speed of lava release would allow Kakashi time to avoid it, though I'm interested in how he would combat acid mist. If the fight were inside an enclosed area, it could spell doom for Kakashi, but if it's out in the open, there's ways for him to avoid it and counter attack, again going back to his doton skills. but given her lava release, we can assume she's capable of digging underground just like Kakashi, given lava release requires doton.
Kakashi's best bet is to engage in CQC right off the bat in order to avoid turning this in to having to fight lava and acid mist. Bunshin feints would definitely be key here, IMO.
Restricting kamui, Kakashi's best way to win is like I said earlier, engage her in taijutsu and defeat her that way. Mei's best way to win is to keep her distance with using acid mist and lava, and if neccesary, using Hiding In Mist.
Allowing Kamui, if Mei has no knowledge of it, it could get tricky for her. She would most definitely need to use Hiding In Mist at that point, to nullify his sharingan, though we have no clue if she's skilled in silent killing or not, so it could hurt her just as much as Kakashi at that point, maybe even her more if Kakashi can get her scent to his nindogs.
It'd be an interesting, almost 50/50 fight, IMO.