When someone Tweeted at the Rocket League Help Twitter asking about cross-platform party support, the account responded with a confirmation:
"Hey there! Weere actively working on cross-platform party support for a 2018 release. :)"
When asked by others for additional details, the account goes on to explain that there will not be cross-platform item trading, but it's unable to provide any details other than that.
Developer Psyonix already created cross-platform support, by letting console players play alongside PC players via a system fans know as Psy-Net. Though currently, this does not include the ability to create parties with players between platforms.
Expect more details about this feature at a later date, quite possibly alongside even more surprise features.