I think that Whitney Bjerken is the most popular. She is very clean and has loads of potential on beam but is not there with difficulty on the other events because of being out for a year due to elbow issues. It's hard to say about her right now but depending on how lucky she is injury and mental block-wise, I still think she could just make elite, in the later qualifiers of 2019. National team would be a stretch though.
I don't know about Sydney Morris as much but she has at least made elite, so that's quite the accomplishment already. But if you objectively compare her championships results last year with those the same age as her in the same competition, it honestly wasn't that impressive. There are tons of kids out there who aren't as popular that I would see on the national team before.
I think they are the two most popular YouTube gymnasts with a chance at/doing elite. Instagram is a whole other story though. Sooo many instagymmies out there who have their Olympic careers all planned out. It's insane and I take it all with a grain of salt.