Aside from the three new maps, the dataminers have discovered a possible hint that the One Year Anniversary Event will be featuring a new game mode.
The discovery points to a unknown character called Athena saying "Welcome to the Arena," and other arena-related terminology, like Challenger and Gladiator. Players of World of Warcraft's Arena system will recognize that Challenger and Gladiator referred to specific ranks in progression.
Perhaps these "Arena" maps will be a sort of King of the Hill style match, where players duel each other, and see who can last the longest, with Challenger and Gladiator being ranks attained for lasting a long time.
Aside from that, there are also lines from Overwatch character that talks about "obtaining items". They talk about gifts, new gear, treasures, leading us to make an assumption that probably the new event will rewards players with special items. Here are some of the voice files:
Reinhardt - What Magnificent Treasure is This?
Winston - Ooo Ooo I wonder what I got?
Soldier 76 - I could use some new gear
There's no assurance that this will be a new game mode but, it's safe to for now that the upcoming One Year Anniversary event will be full of awesome surprises. Stay tuned for more updates about the upcoming event.