Sometime in October, Psyonix plans to roll out a Steam beta test for cross-platform parties. While it'll be confined to just PC, the goal here is to a) load test for eventually scaling up to handle 100 percent of Rocket League's users and to b) implement the game's own friends system.
Psyonix doesn't have the full-on green light, though. There are a couple of hangups which may potentially never get resolved. For instance, Davis says that he can't yet answer whether players will be able to form PlayStation to Xbox parties. "We're still talking to partners," he said. Between Nintendo, Sony, and Microsoft, it would be hardly surprising if one or more of them flatly refused to allow cross-platform parties with their direct competition.
Similarly, it's unclear how a chat system might work (especially given how convoluted the Switch's voice solution currently is). For now, Psyonix says that it's "working with partners regarding what restrictions would need to be in place for cross-platform friends." Also, it still seems as if trading items across platforms won't be an option. Psyonix's stance on that right now is "We don't have any current plans to support cross-platform trading primarily because our partners have policies that forbid trading most of what players would want to trade."