Let's be honest. We can see that Lapis Lazuli has a high chance of joining the team. I mean, her concern for Steven is just so strong that she fused with Malachite and trapped herself in the ocean for him.
Why will Lapis Lazuli join the Crystal Gems in Steven Universe? Here are some reasons:
1) Lapis Lazuli's Payment to Steven
Technically, this isn't money but protection. Lapis provides Steven protection. Why? He freed her from the mirror and another thing, he helped her go home (which caused a change of plan in Steven's Birthday [I could've seen Ruby and Sapphire fuse with other Gems]).
2) Lapis' Concern for Steven
During the episode "Mirror Gem", Steven had created a friendship with Lapis through the mirror. The two seemed to have a bond. Steven called themselves, "Beach Buddies!" She saved him from an injury by warning him of Mayor Dewey's van, which was supposed to hit him.
3) Lapis Lazuli's Revenge
We're still not sure if the Crystal Gems really trapped Lapis in the mirror or if Lapis had just mistaken them for it. Clearly, Pearl said, "We found this Gem-powered mirror at the Galaxy Warp. " Lapis will find for the gem who trapped her in the mirror with the Crystal Gems.
4) Lapis Lazuli's Gemstone
This is what I observed with all of the gems. All of their gemstones are circular. Even Peridot's gemstone has rounded corners (though I am not sure about this sub-theory, since Blue Pearl and Yellow Pearl's gemstones are circular). But what do you know? Yellow Pearl might join the team for her Diamond's austerity.