At least one eyewitness. It was reported that an earwitness was comforted by investigator Serino saying that person yelling for help survived.
I think John will testify; I don't trust the 13 year old to give truthful testimony. He may have played football with Trayvon, and in the "snitches get stitches" category, his peer group is black males.
But yeah, on Sybrina's role in Corey's fiction - Sybrina is counterbalanced by Zimmerman relatives who will testify to the opposite of what Sybrina does. So, all of those interested parties "balance out" as useless. We're left with the same evidence that SPD had, which, ironically, includes Tracy Martin saying the screamer isn't his son.
-- Since Dee Dee didn't initially come forward...My first question is "what is she hiding?" --
Probably she's hiding the truth. She knows that Trayvon was playing cat and mouse, that he wasn't scared, and she may know that Trayvon decked Zimmerman.
At any rate, coming out after significant parts of the investigation are public gives her time and information with which to craft a plausible lie. Further, she's an interested party.
-- How difficult to prove witness tampering/intimidation? --
Snitches get stitches - pretty difficult to trump that.
But you can't intimidate a security camera. Ergo hoodies.