...Or is it a crossover of those worlds? It's hard to say.
In any event, the idea of Marvel Zombies and The Age of Ultron playing into whatever it is that Marvel is teasing begs the question: is there some time- or alternate-world-travel associated with their upcoming Secret Wars event? Obviously the teaser image released for that mega-event featured "classic" iterations of Marvel heroes seemingly battling their more contemporary counterparts.
That implies either a time-travel or multiverse story. Perhaps in the absence of Uatu The Watcher following the events of Original Sin, some cosmic threat finds messing with the boundaries of space and time to be easier to do without being noticed.
Either way, Age of Ultron vs. Marvel Zombies -- the most Hollywood-friendly combination imaginable, featuring the title of the next Avengers film and a story written by The Walking Dead's Robert Kirkman -- will hit in the Summer of 2015, just like the Civil War story they teased. More as this develops...