I want him to join SHs or SH alliance so we can get more panel of him....
He's a true adventurer
Oden flashback showed how similar he is to the SHs in character about going around the world...etc..
However, he's a huge walking spoiler.... cause he's been with WB.. and Whitebeard probably knows waaaay too much...
My assumption which I'm hoping it will happen at least is NOT join SHs... but rather, Marco will accompany them TEMPORARILY like Law did to only drop him off on Whitebeard island after Wano arc... So, we may actually just get 1 small arc between Wano and dropping him off... or maybe the Anime will give filler arc for it...
My reasoning is that I can see Marco after Wano... when they drop him off that island... to ask Luffy to come because he wants to show him something...
Luffy goes with Marco and both go pay respect to WB and Ace grave...This is a scene I firmly believe Oda will deliver... ESPECIALLY since there is big ties between Wano/Oden/Otama to WB&Ace
It is the perfect time for Luffy to visit their grave and pay respect to them... basically freeing them from their promises during their lifetime