There are 8 legendaries that have yet to get an EX. Suicune, Regice, Regirock, Uxie, Mesprit, Azelf, Manaphy and Arceus. More if you count Phione (who, while I do actually consider it legendary, won't get an EX) and the alternate formes of Deoxys, Giratina, Shaymin, Meloetta, Landorus and Thundurus. Japan has EX's for Latios, Latias, Jirachi, Deoxys, Heatran, Dialga, Palkia, Virizion, Thundurus, Genesect and Meloetta, and the rest exist worldwide.
I don't think Arceus will get any special treatment if it is released. I think the fact we haven't had any Arceus since Platinum: Arceus shows their reluctance at doing another Arceus overload. Actually, they haven't done any Rotom or Unown formes for quite a while, either. (I mean all of them, the Plasma Storm Rotom doesn't count. >.>) I think they've stopped doing subsets of specific Pokemon, and I don't blame them. Arceus-EX will likely just be a pretty good standalone EX.