Unfounded speculation, but is it possible that every particle could have a different individual light speed, and the one we know is just the average speed that they have to drop or speed up to, the way a car needs to travel at the same speed as the highway?
By "every particle" do you mean like, every type of particle?
If you mean individual particles, fundamental particles don't really have distinct individual identities (as shown by fermi and bose statistics).
As for types of particles: Well, photons surely move at the speed photons move at.
Special relativity is derived from the assumption/observation that light travels at the same speed in all inertial reference frames, and generally that the laws of physics work the same in any inertial reference frame.
What you are proposing sounds pretty vague and unclear to me, but, is what you are trying to say compatible with this?
Probably no, because according to quantum mechanics, photons are indistinguishable in a way that affects experiments.
What measurable predictions does this hypothesis make that would differ from the existing models?
Well, yeah. Perhaps if this experiment was expanded for a chain or a filter of cause and effect then it could be shown that one event affects another.
As opposed to current models, where one event does not affect subsequent events?
Good god, sorry I participated in any way. Apparently there’s a quantum physics colloquium I’m disturbing.
I'm sorry, I thought you knew how scientific ideas were discussed, I didn't know you were just trying to bootstrap a new religion.
Before you figure that out, you will be surprised to discover that it's impossible to tell if the speed of light is the same in both directions, with only the average of a round trip being the speed of light.
See: https://en.wikipedia.org/wiki/One-way_speed_of_light
Anyway, reading that should help you answer your own question, or at least give you a lot more questions to ask.
Criticism: It's not literally impossible.
I hate this one because it's true Einstein said we cannot measure the speed of light in one direction that is independent of the clocks' synchronization technique being used to measure.
And yet, somehow we changed that to "we don't know the speed of light in one direction!!!" Which is bogus - can you imagine if speed of light was different if you were pointing east vs west? In space, what is even a direction vs another?
Did you even read the wiki you linked? We've done one-way speeds, it's down below.