I've played the first Destiny lately in short sessions, just doing random missions around the world. Despite already having done some of those missions in the past (my quest log's all kinds of messed up), I've still had fun replaying them just because I like the gameplay a bunch. If there's a solid campaign in two that tells a coherent story within the game, that alone might be enough to sell me on Destiny 2. Obviously I wouldn't mind them polishing it up in several different ways as well, but that'd at least be a good start.
I've got the beta downloaded on my PS4, so I'll be in the first wave of testers. That is, if we're assuming that the servers won't blow up.