Regarding the new knowledge that Nintendo has decided the characters for DLC instead of Sakurai, I don't really think the likely hood of Joker has changed at all, if anything it may probably be even higher than it was. I think a big thing that might hold Sakurai back from adding Joker is just that he doesn't really hold any presence in video game history. Every third party thus far (With the exception of Bayonetta) are very iconic and hold a lot of history. Nintendo however I don't think has much interest in adding characters based on their history, they're a business, and right now with Persona Q2 coming out and possibly Persona 5 on Nintendo Switch (Which really needs to happen please), they have fair incentive to include him.
However, there is something that interests me regarding
Sakurai's wording in his tweet, to me it sounds as though Nintendo gave Sakurai a selection of characters to pick from, to which he then picked which ones he's interested in including. If this is the case I think Joker's chances have skyrocketed, if Nintendo included him in their selection I'm positive he's one of the characters Sakurai picked.