Blizzard's Jeff Kaplan replied to a forum question asking about the future of Overwatch, explaining in detail what they are working on and where their focus lies for the multiplayer shooter. He discusses the incoming competitive mode, various interface and other quality-of-life changes they plan, and without going too far into specifics, discusses plans for new heroes and maps. Here's part that covers those plans for new content:

We're also working on new heroes right now. We have some that are very far along and others that are just getting started. We also have some prototype heroes that may or may not ever see the light of day. We do a lot of prototyping... I don't think we're at a point where I can really talk openly about the release schedule for the new heroes because I don't want to set expectations and then have something come up and suddenly we're backing out on a "promise" we never really made... if that makes sense.

We're also working on a few new maps right now. One of them is "officially" in our production pipeline as it has passed through enough prototyping and playtesting that we feel confident in it. So that map is in the hands of our art group at this time. They are busy making it look awesome and while they do that, we keep going back and playtesting it to make sure nothing changed between the early level design blockout and the final "artified" version of the map.

We have a few other maps that are in prototype and playtest phases right now. Some of them will become real maps and some of them will be totally scrapped -- so I can't really commit to you on those. For example, we tested a map yesterday that was amazing yet a total disaster at the same time. Aaron tried some very innovative level design tricks that we hadn't done before in OW. On the upside, the map was very unique. But the we ran into problems with sight lines and the map was basically dominated by long range heroes, leaving heroes like Reaper practically useless. So if we can fix the sight lines while not losing the overall vision, that map might turn into something some day.

So maps and heroes are being worked on daily. I wish I had dates for you but whatever date I gave you would change and then you'd all be mad at me. The good news is, like we've said before, for Overwatch, we won't be charging for those maps or heroes. Those will just be live updates that will come in a patch for free.