Out of curiosity, I decided to look at the ID numbers of each pack, not the SWX/SKU numbers, and I noticed something :
SWX55 Protectorate = ID token 59/60
SWX56 Shadow = (special) ID token 1-3
SWX57 HOTR = ID token 43-44 (according to wookiepedia http://starwars.wikia.com/wiki/Heroes_of_the_Resistance_Expansion_Pack)
SWX58 ? =
SWX59 Sabine's Tie = ID token 64
SWX60 Upsilon Shuttle = ID token 65
SWX61 Quadjumper = ID token 66
SWX62 U-wing = ID token 61-62
SWX63 Striker = ID token 63
Seeing as there are no gaps in the ID tokens between 59-66 (and I went back as far as Wave 8, with ID tokens covering nos.48-58), could this indicate that SWX 58 is NOT an expansion pack of some kind? Perhaps simply a game accessory/mat as some have predicted?
Then again, it could be a new core set, seeing as they have a completely different numbering system (Core set = 1-9 and FA core set = 1-6)
Or I could just be barking up the wrong tree and should just be patient....