Perhaps. But that doesn't really explain why Jigglypuff got in over Mewtwo in Brawl, I think. Considering how close to completion Mewtwo was, you'd think they could have ditched Jigglypuff in favor of bringing Mewtwo back. After all, Mewtwo was still a very popular Pokemon at that time, and technically more significant than Jigglypuff. The fact that they kept Jigglypuff despite that means something, I think.
At the time, probably.
However, developers change their mind all the time, and Smash is no different. Priorities may shift, and other companies might intervene. Sakurai considered the Villager being a feasible fighter now, and I think people would tell me that's it's a bad assumption if I say Forbidden 7 characters will be planned again. Sure, Sakurai thought about returning Dr. Mario in Brawl, however, he may not even think about him in this installment. Heck, it will be a big assumption if I say Roy will be planned again, don't you think?
We don't know who will be high or low priority right now, and we don't know if Mewtwo and Jiggs will have the same fate as in Brawl. However, leaving Jiggs until the end as an easy/lazy character to add in does make sense from a technical standpoint.
They seem to be hyping Smash 4 a lot -- I'm not sure if Sakurai could afford one or two delays. Yeah, I'm hoping for DLC characters... at the very least, I'll be very disappointed if we don't get any DLC, especially in this day and age.