The Kokuto Yoru Mihawk is holding won't go anywhere, lol. I don't think it fits Zoro at all! For him, it's gonna be a katana, even though it's been long clarified that he's not a samurai. Recall him using Johnny and Yosaku's swords, ugh, it doesn't look good to me.
True, I am thinking of that too a lot, how the heck is East Blue the weakest sea? But maybe, it's not particularly because of the overall strength of the sea, but since it's more of a peaceful sea thus getting labelled the weakest. Looking back at One Piece revealing of the other seas, West Blue has the Ohara incident, North Blue has Flevance and the Germas, these two so far has really ground-breaking events, compared to the much unknown South Blue, where Ace was originally born and of course, East Blue who only has Loguetown event their best twist in the whole One Piece history.
I kinda agree with you that it should be a 10 Strawhats and 10 BBP battle. But if we take Oda's words when he said that there's gonna be a new nakama who's an enemy before and Luffy's choice from episode 1 of getting 10 nakamas, excluding himself. It's safe to say that the final Strawhat roster will 11 so it'll also apply to the BBP getting another nakama in the future for their numbers to match up. Setting aside Kuzan, who's currently Teach's ally for some reason, it's obvious Luffy will get Teach, Zoro versus Shiryuu, Jimbei will get the impossible match-up in Sanjuan Wolf given they're both water creatures, but the other matchup remains vague, as Usopp should handle Van Augur, but Vasco Shot is also a gun wielder. Sanji should be perfectly matched up with Jesus, but there's also Avalo Pizarro, who seems like he's a mano-e-mano type of fighter. Nami or Robin against Catarina Devon, Laffitte vs Brook, Franky vs Jesus maybe? Chopper vs Doc as they're both doctors? Ugh, I'll leave this topic as it is.
Going back to Kuzan or Marco joining the Strawhats, I'm kinda inclined to see it's Kuzan who gets to join. He's far stronger, he's also a great fit to the already-weird crew of the Strawhats and he's somewhat indebted to Luffy's granddad, Garp. So I really want him to join the crew, but again, like aforementioned, this is almost 0% happening. Kuzan seems satisfied with his lone-wolf status and he also looked like he's on a personal mission, so Marco looks like a safer bet. I can safely say Marco and Luffy are on the same power level now, maybe Marco is a tad stronger though. But by the time, Luffy invites him to join his cause of defeating Blackbeard, Luffy will get much stronger, and so is Zoro and Sanji to maintain their Monster Trio reputation. Marco will be the fourth strongest dude in the crew and he'll be the perfect dude to add to their roster. But again, with all the points you said, him joining Strawhats right now is wishful thinking.
Again, taking account Oda's word of the next nakama being a previous enemy, I can only think of these following characters:
Bon Kurei - I love this dude so much. He's also a very close friend of Luffy and the crew, but him joining as the next nakama is plain impossible. First, he's imprisoned at Impel Down 5.5, second, his strength level won't be a plus to the upcoming battles and third, him and Franky shares the same VA's so it's gonna be weird to hear them both say their lines simultaneously.
Pekoms - As of this current arc, Pekoms looks like he'd be a great addition to the crew. From the fact, he's a weirdo himself, he also holds great gratitude to the Strawhats. The only factor preventing myself to give in to the idea of Pekoms joining is his loyalty to Big Mom. It's hard to see Pekoms leaving the BMP for Luffy's crew. His power level would be a great plus though.
Caesar Clown - he almost looks like the next Strawhat member. His strength level is close to the top-tiers and he's a great weirdo too. Of course, there's his annoying and evil personality to take account, but CC already looks like a great addition to the crew.
Pedro/Carrot - as they are not really enemies of Luffy, it's safe to assume they do not belong to this category. They are strong though.
Sir Crocodile - I've read this on some other forums. But heck, this dude won't allow himself to get bossed by Luffy no matter what.
Still, Marco or Kuzan remains my favorite to be the new addition, as it will greatly boost the overall raw strength of Luffy's crew.