I don't actually want it to be Krypto at all. That would be a hell of an anticlimax. I'd like it to be something that comes as a surprise to the viewers, rather than just being the exact thing they've been speculating about for four months.
I can understand that. Trouble is, most of the other reasonable possiblities (other Kryptonians, Daxamities, Brainy) are out IMO because J'onn didn't recognise the 'feel' of whatever was inside and he's obviousness familiar with the feel of Kryptonians and somewhat (thanks to Indigo) familar with Coluans). I'd also say that a Durlan might expand the mythology of the series, but given that their only "hat" is shapeshifting wouldn't be much help narratively.
Personally, if they're going for another alien, I'm hoping for a Xudarian like Tomar-Re and Tomar-Tu of the GLs (actually one of them would be cool) as Xudar is apparently one of the few planets other than Krypton that support(ed) sentient in that Sector.