If the MCU really wanted to be creative, the symbiote goop left behind should turn out to be Mania.
But as much as a nerd flex as having Not!Venom is, that’d be a bad idea. So instead of Eddie as Venom again they should either jump straight to Venorpion or Agent Venom.
Mac Gargan has been teased since the first movie, so folding him and the symbiote together would be great to condense plots when you have to constrain yourself to a movie format. After/failing that, Flash has been used throughout the trilogy. His fanboying of Spiderman would be a perfect setup to giving him the symbiote, thus elevating him to main character status.