This is just a list of my best ideas marketing wise for the next Spider man 4 movie. A little while ago it was rumored that the Vampire Morbius would be in the movie. In my opinion there is another better pick Carnage / Cletus Cassidy. To increase sales Marvel will probably pick villains that are current ly popular or are similar to pop culture fads. The two largest fads in my opinion are Vampires and the deranged insane villain shown most popularly in the Twilight and Dark Knight franchise. For some reason the insane villain has sparked the interest of movie goers. In my opinion there is no darker and crazier than the serial killer turned supervillian Carnage. If marvel players up the darkness of the villain and manages to keep it near the border of R rating but still PG 13 - much like what the Dark Knight did- and keeps it fairly true to the comics they have a guaranteed hit. They should be able to draw in the fan boys , average film goers and possibly get some fans of darker works. They might even be able to get some back draft of the Dark Knight if they increase the craze appeal. Also second to Lizard man this is one of the more memorable villians of the spiderman series.
As for potential actors for the role of Carnage I think a great pick would be the actor who played Rorschach in the Watchmen film. He played his part of a insane hero/ anti hero amazingly and also has the right build and hair color for the role.
If you don't know anything about who Carnage is look up a 101 on youtube. This article was just my thoughts and do not necessarily represent the vies of Marvel Comics or their distributors