According to the French Le Journal de Gamer and a confidential source of theirs Half-Life 3 is going to be an open world game inspired by the likes of The Elder Scrolls V: Skyrim. While it's still going to be an action game, there will be people to visit and talk to, side missions and such things to get stuck in with.
Even if the source is said to be reliable, you should probably regard this rumour with a healthy amount of doubt. Valve have been keeping very quiet on the subject of Half-Life ever since Half-Life 2: Episode Two came out 5 years ago.