Unless this Elder is deep undercover posing as a member of the pirate gang, him as an Elder seems unlikely.
The Collector being an Elder matches up with the comic book continuity, so we’re not surprised to hear that he might maintain this status in the MCU.
As for Sharon Stone, she mentioned earlier this year that she has a role in an upcoming Marvel movie, and has so far only provided two clues about her character: she’ll be a good guy and will have a special power tied to heat.
In Sylvester Stallone’s case, his character was seen in the San Diego Comic-Con footage dressed in a Ravagers uniform.