In what way is it a raid if it's done solo? The very definition of a raid is multiple people exceeding the standard group size laying siege to an enemy. If it's soloable, it ceases to be a raid.
SPECIAL solo dungeons I can get behind, maybe akin to Maat from XI. DCUO did these types of things and it works pretty well. Basically imagine harder versions of our Job quests. The rewards aren't great, maybe no more than doing a map would be (Maybe it could be the new form of alexandrite maps!), but they're still prizes.
This is a multiplayer game. There is and will always be plenty of solo content, but there should ALWAYS be things locked behind group content, otherwise this would just be a single player game.