Diablo IV

[Frag]

Will there be both solo play as well as online multiplayer?

It's hybrid - it's a persistent online world, but you only see yourself (and your party, if you have one) in quest areas until you've completed them. After that you'll encounter other players as you return to those areas.

Bluntly, sounds like phasing tech from WoW applied to an isometric-styled game.

To answer the possible underlying questions you bring up - no offline play, and no modding.