I am aware that listing ( this character ) as a member of rocks is pretty popular now but I think it makes sense that big mom,kaido and whitebeard were at some point crew members. Now my biggest hint at this speculating is the big age gaps between the captains and the core crew members. Let’s take for example Marco- whitebeard, katakuri- big mom and jack - kaido. With kaido it is hard to say because we can’t really tell how old he is nor how old jack is but it was establish that kaido and big mom go back a long ways so he has to be above his 50s or 60s maybe more since I think he is a giant ( I heard many arguing he might not be since big mom is a human and also pretty big ) anyway with his crew we haven’t really seen his core group just low rank members who could have joined recently. Jack is the only one who we seen so far and jack again is hard to tell since he isn’t human but I say hen is around 40. Now with big mom she is 68 while I believe katakuri who is her 2nd son along with his 2 triplets are 40. Finally whitebeard who died at 72 and would be 74 had Marco who was also around the 40 mark when we last saw him. So this means that the captains have about 20-30 years difference between their top commanders. Now luffy is pretty young compared to his crew. Nami Sanji and zoro being a year or 2 older than him while robin and franky were already kids when he was born and brook is 90. Chopper is the only one younger than him. Though it is different from recruiting people who are older than recruiting younger people. Now with whitebeard was said to have been Roger pirate rival throughout his adventures so whitebeard was around for a long long time so at some point you will have to ask who his crew was. He couldn’t have had the same crew he has now since Marco who was the same age as shanks would have been another adolescent on the crew and I don’t see whitebeard having a crew of teenagers. We know in a flashback that he did sail with another crew before and he wasn’t a captain then. So the 4 main options are 1. He was a solo pirate, like Hawkeye who doesn’t appear to have a crew. 2. He sailed around with a bunch of kids who were strong enough to atleast take on Rogers crew members like reyleigh 3. He had another crew who died or retired as pirates and marco and the others are the 2nd whitebeard generation of pirates. 4. He sailed with another crew and started his own crew years later after they disbanded. Now I just don’t see whitebeard sailing alone as a pirate as he seems more like someone who wants to be around other people, I also can’t see him sailing around with a bunch of teens for a number of reasons. As for 3. I think this is possible but by now I think we should have heard something about the former pirate crews. Also being a pirate isn’t really something you can retire from. Reyleigh retired only after Roger died and even then he went into hiding because his bounty was still up. So you would expect whitebeard old crew to still be around unless it’s like luffy where he recruited much older crew members who died before the serious. I think it is more likely he was a commander of shorts among another crew, now it is still a long way away from the end of one piece but I think it is too late to be adding even more lore to this world about the previous life’s of someone who has been dead a few years now. The best opportunity to make whitebeard past life relevant is by including him into the rocks pirate. Having 3 future yonko as part of this crew would certainly make this rock guy legendery as well as showing off Roger and garp strength. Another feature I would like to take note of is the rivalry between Roger and whitebeard. If whitebeard was just a commander why would he and Roger a captain have such a strong mutual respect and rivalry between them? I think we have seen this same bond starting to develop recently between katakuri and luffy, Katakuri nearly killed luffy and brought him past his limits while luffy was able to fight katakuri head on and come out victorious. I don’t see katakuri becoming irrelevant anytime soon he will always be a match for luffy but it will become more clear in the future that luffy will be the stronger 1 between them. I believe katakuri will be luffy’s pirate rival like whitebeard was to Roger and how smoker seems to be luffy pirate rival similar to garp and Roger. So i believe the same can be done with the rocks crew, whitebeard might have been a commander but he could have still been the crew member closer with Roger. Finally i would like to mention something else which isn’t really beneficial to this theory but is something I think is interesting nonetheless. Whitebeard shares some common traits between the 2 yonko. With kaido both have or had the title of the strongest. Whitebeard being the strongest man and kaido being the strongest creature. With big mom it is the family. Whitebeard was the father of his crew while big mom was the mom of her crew.