to say that all omegas are believed to transcend past the human form is not the same as saying all those transcend are omegas. the shadow king, for example, is not a confirmed omega, though he exists solely as an astral body. jeffrey garrett of the new x-men: academy x series exists as some variety of living ghost, though he's not a confirmed omega. king bedlam once said about paradigm, a techokinetic mutant whose body has become irrevocably fused with phalanx matter, "i imagine one day he'll no longer need to maintain a human shape." thunderbird III becomes living solar plasma. saying transcendence is a trait of omegas is not false if there are non-omegas who also transcend.
I'm not saying that this is false or true, but I'm saying that there isn't enough evidence to prove or disprove this. The same could be said for any number of similarities between these four. For example:
it's a common trait of omegas to evolve past the human form as they ascend toward their full potential.
it's a common trait of omegas to be caucasian.
Because something is common doesn't necessarily mean that it's related.
rachel being an omega's also generally speculation at this point, though is likely the direction marvel will go with her. her powers were jean's and then some during her early days, but this was before the omega concept was brought to the surface, and long before the recent changes to the phoenix status quo. rachel comes off as a bumbling novice these days, despite having been extraordinarily powerful during excalibur, and despite the now-erased askani timeline. where they go with her is up to the writers, though it would seem to make sense to have her be yet another confirmed omega. whether cable or x-man would fit those classifications (though likely) isn't proven. legion was a walking god on the face of the earth, but that doesn't necessarily classify him as an omega.
In both the old and new Marvel Handbooks it's been explicitly stated that Rachel inherited her mother's powers. Endsong says that it's the province of omega TPs to wield the PF. Rachel was Phoenix. Emma couldn't contain it. I can't think of a better textual example. Bumbling idiot or not, Rachel has the power, and according to continuity, she should be considered an omega. And she hasn't transcended anything yet.
franklin, eh, what's the point in speculating? he is a mutant, he's one of the strongest expressions of the mutant gene out there, and his hypothetical future kid was ten times his superior. though noone's out and out said "franklin's an omega," every other possible terminology to infer such has been used.
What is any of this but specualtion? I don't doubt Franklin's power. I just question why he's called what he's called. If homo sapiens will be extinct in a few generations, would we call the people that live then mutants? I wouldn't, because they aren't mutants at that point. But really, that's neither here nor there. And as far as people calling Franklin an omega in everything but name, the same can be said of Cable, X-Man and Stryfe.