Yeah, people seem to not remember, but the first one of the "Episode of" series of specials had its character designs done by Nakatani, a member of Guts studio, and a staple member of the One Piece AD rotation back then.
Of course, despite having the character designer and chief animation directior technically be an outsider of Toei, the special itself had few of Toei's animators, including the likes of Shingaki, Urata, Kuroyanagi, and Ishizuka.
Admittingly, that special had way less animators from Toei, but that was mainly due to giving many of the key animation to other studios.
Obviously, this might not be the case for the upcoming special, but one can only hope. And I actually think it's more likely to have more outsourcing than not.
Currently, what concerns me the most are the directors, because sometimes these specials had up to 5 storyboard artists including some of the ones working in the show, but if the upcoming special has a better planning, it'll probably be just one director.
Regarding the special itself, I've high hopes for whoever does the character designs for the special, who'll most likely be Isouchi. And despite his occasional extreme proportions for females, I think he can deliver pretty decent if not great character designs. Btw, his style seems to be immensly similar to that of Ichikawa's, which I believe a considrable number of people here noticed. Actually, now that I think about it, this could probably be the main reason they chose him for the special, especially with Ichikawa's style being highly regarded both here and in Toei itself.
–- Update From New Post Merge ---
On a side not, personally I hope the special will have an original story instead of just readapting an old arc, primarily because of pacing issues, but also because I can't think of an older arc other than Skypia, Thriller Bark, and Marineford, last one being something that will require them to pull all the stops, of course that's if the new story doesn't portray some of the Strawhats bafly like Heart of Gold and isn't a snorefest like Nebulandia.