I didn't realise all of D2s content had been announced. I must have missed that press release. Bungie won't have released details of all content, and from what they've mentioned so far there is a lot of more of the same, but they've also introduced more activities in patrol space.
Completed all the uncharted games, and generation leap aside that provided graphical improvement my point stands. The general mechanics were the same with small changes and improvements in each iteration.
So adding a new campaign is a prerequisite for a sequel? Pretty sure that D2 has a new campaign / story mode which will probably be as long as most games released this year. Hell D1 "story" was longer than most COD campaigns. The upcoming uncharted expansion has a new campaign. Is that a sequel? It looks like more of the same but with different characters.
D2 was always going to look similar to D1. It's set in the same universe. Yet the locales are different, the hud has changed, character abilities have changed, odds are the UI has been overhauled as well. But again go back to the uncharted games, they had slight graphical changes over the years. But if you were shown screen shots from the games randomly most people would identify it as uncharted, they would struggle to identify which specific version it was from.
As for your point of U2 having multiplayer making it a sequel as well. It's not like multiplayer modes can't be patched in after release is it.
The only reason people are calling destiny out on the sequel / expansion thing is because of its MMO links. Yet if you apply the same points to any game series you can argue the same point.