Yesterday’s Hyrule Warriors-themed Nintendo Direct featured a lot of information (check out a summary of all the major points here) but didn’t delve into whether there would be an online mode.
An interview with Game Informer this month, however, revealed that there will be some sort of online mechanic. According to Koei Tecmo Producer Yosuke Hayashi:
The Adventure Mode features a ‘Network Link’ feature. This allows you to add a ‘Link’ to your maps whose level is based on other users’ play data. If you come to this Link’s aid and fight alongside him, you’ll get access to special bonuses.
So while it’s probably not the online multiplayer or co-op many fans wanted or expected, the use of Ghost Data in Adventure Mode is certainly an online interface.