The Division has improved massively since launch with some crucial tweaks and new ideas. The original gameplay director left soon after release to join Square Enix and he really didn't have a clue on what the game should be and how it should move forward. Thankfully Ubisoft have supported the game closely ever since, worked with the community and improved the whole experience. A sequel is now a no-brainer as the brand has been somewhat salvaged.
The problem with the original was down to how they mislead people on how the game worked and how exaggerated the marketing was. The dark zone was good on paper but poorly executed and the fact you didn't see any other players in New York aside from you and your party just made the game feel bare apart from repetitive enemies dotted around. It wasn't really the MMO, multi-dark zone open world survival game that people were expecting and that Ubisoft had promoted. The game didn't really have to be online only either. I'd also want more of a story this time around rather than a load of missions and activities simply slapped over a huge sandbox environment. The game world did look astonishing at times though with incredible attention to detail.
If the first game didn't lose so many players I think they would have just continued expanding it but a sequel is probably best for a refresh, to bring players back and hopefully a totally new location.