We know that Theros will have a rich variety of different creature types, not only because its ancient Greek source material is full of various fantastical beings as well as ordinary people of all stripes, but because of the many creature types that came up at the San Diego Comic-Con Magic panel this year - Griffins and Gorgons, Minotaurs and Centaurs, Satyrs and Krakens. We also know that there are some creature types that will make their debut in Theros, because apparently we'll be seeing creatures that are relatively unprecedented in the history of Magic. But what will these mystery creature types be? Let's see if we can speculate.
- The monstrous: There will naturally be a whole gruesome menagerie of monsters, since they are one of the themes of Theros, but one would think most of the characteristic critters of Greek mythology are covered by current Magic creature types. Even Harpy became its own type via errata in the Grand Creature Type Update. Regardless, there are probably some relatively obscure monsters that R&D would like to print some of and that wouldn't be done justice by preexisting types - the Hekatonkheires, for example, are hardly mere Giants, and the Bonnacon and Catoblepas really show the poverty of description of the word "Ox."
- The heroic: It's hard to think any professions per se could not be represented by existing "class" types, but I could see Philosopher being added as a creature type - ancient Greece had a lot of those, Meletis certainly does on Theros, and the idea of being a Philosopher doesn't quite fit precisely with being a Wizard or with an Advisor. Speaking of Meletis, there seems to be a lot of action going on in that harbor - maybe we'll see Merchant reinstated as a type, or Sailor creatures for law-abiding non-Pirate seafarers.
- The divine?: I will come out and say this right now - we won't be getting the word God on the type line of any creatures. The five major gods of Theros will be represented as legendary Avatar creature cards, because, really, that's what the cards represent - the physical manifestations of an untouchable, unknowable, immortal being, which the god can switch between at whim. It's also a bit ignominious to think that a god in his, her, or its true form could be destroyed or returned to a hand or shuffled into a library or otherwise removed - and giving all of them indestructible and hexproof would make for boring designs and frustrating gameplay. A similar excuse for the vulnerability of the three legendary Eldrazi (the creatures represented on the cards were actually Zendikar-specific forms that the colorless fires of Ugin had bound the Blind Eternities-dwelling monstrosities in). In spite of that, Elspeth was mentioned to have been drawn to Theros and led down her path by "the Fates" - if this isn't metaphorical speak and there are actually beings like the Moirai in Theros, then I could see Fate being its own creature type, since they are so different from the gods and indeed from any other beings.