I guess it depends if the new Rated-R Wolverine movie does any good in the box office or not. But realistically, it would have to be a VERY SLOW build to ever get to Carnage.
During the Homecoming movie, introduce Eddie as a reporter for the Daily Bugle. He's not great by any means. Just a side character.
<Spiderman takes part in Infinity Gauntlet, where he encounters the symbote while fighting in space. No reason to introduce Jameson's son as an Astronaut that brings the suit back to earth that way>
In the next movie, Spider-Man suddenly starts wearing the black costume Maybe they didn't have to introduce the Symbiote in Infinity Gauntlet, but this movie could do a "flash back" showing how the Spider-Man encountered it while helping the world. Brock is working on this story about a serial killer known as Sin-Eater. He has this guy that claims to be him, and Brock takes the story public. During the movie as a side scene, the REAL sin-eater is captured by Spider-Man, thus showing that Brock had it all wrong and he gets fired. The main villian could be Kraven.
The next movie, Spider-Man realizes the suit is no good, so he finds a way to rid himself of it. Meanwhile, Brock is still brooding about Spider-Man, and constantly follows him around, hoping to catch him doing something wrong so he can regain his reporter cred. In the mean time, He's getting ripped as a body builder. When Spider-Man rids himself of the suit, it is drawn to Eddie because of his hatred for spider-man. How about Green Goblin is the bad guy for this one.
The NEXT movie can finally have venom. At the end once Venom is defeated by spider-man and placed in jail without the symbiote, introduce Kasady as Eddie's cell mate.
Onto the 5th Spider-Man movie. This is going to be a second Venom film because people loved the last one so much. The Symbiote breaks free and goes to the prison to get Eddie out. A little bit is left behind for Kasady to become Carnage. It isn't an instant change though. He's just able to escape prison while everyone is trying to stop Venom. The little piece of symbiote needs time to grow and mature into it's own being.
Movie 6 could finally show the appearance of Carnage as the main villain. It's a very long process (6-8 years if we're lucky), but worth it. It'll probably never happen, or if it does, it'll be a quick throw together, similar to what they did in Spider-Man 3 back in the day. Thus, ruin the entire story and have to reboot the series yet again.