Virendra

I think enel joining is a really high possibility too, shandarians came from moon and were allies of joyboy in the final war( most probably as if you remember in elbaph arc there's that picture of nika fighting a snake in that picture we can see enel's ship so maybe the people live in sky came from a similar ship but from moon to earth) now as enel has gone back to moon in search of endless verth he will know the secrets the ancestors left behind and will eventually side with the current sun god which is Luffy

Just a fun fact if this theory prove to be true then isn't it like enel reached his own one piece (the gold is nothing to him he called moon as endless verth but that endless verth is their birth place and holds all the secrets isn't this similar to what we know if one piece Luffy is finding, there is enough verth on land so one piece would be gold in Luffy's eyes and their goals were hinted in skypiea arc too, remember how enel gone to moon for endless verth which he considers treasure while Luffy for gold so in that sense enel has reached his own one piece and uncover the truths of the world which Luffy will too eventually once reaching his one piece and then enel will most probably become the ally)

What I personally want is Yamato to join the crew but we can't count out any of them both vivi and Yamato

Luffy initially said at the start of the series he wants 10 members in hi pirate crew ofcourse as captain it's upto him and as creator it's upto oda but I think only one of the two will join either Yamato or vivi there is a possibility none of them join too and some other character (old or new) joins but there's only one more member going to join just one as at start of the series I think it wasn't just Luffy's words it's also a message from oda that there going to be just 10 members

Now the most likely members are vivi or Yamato

We all know Yamato always wanted to be like oden and if she wanna follow all of his footsteps currently she is traveling in the land of wano just like how oden started and as we know oden also ended up on roger's ship aka the king of the pirate's ship and saw one piece so it's only natural that yamato see it too and know the reason why oden wanted to open the land of wano and release the ancient weapon

On the other hand we have vivi, as we know she is the descendent of nefelatari D Lily who helped joyboy by scattering the road poneglyph's around the world and as she never returned back to arabasta and her brother (I don't remember properly but that's what cobra said I guess) ruled afterwards so maybe she joined joyboy's crew in the end and if that ends up being true then we can call her the last member of joyboy's crew and as Luffy is joyboy then it's possible that the final member would be vivi there are other reasons too but the one wrote this article already mentioned them so I won't repeat