By this time next month, we should know for sure who the mystery Clark Kent is, so let's use this thread to make our final guesses going into Reborn.
Right now I'm leaning towards Superboy-Prime.
He's a Geoff Johns favorite, and Johns is the architect of Rebirth.
Prime was referenced in an issue of Nightwing, so we know he's not forgotten.
Clark Kent act creepily while believing he has good intentions.
His interaction with Lois, basically demanding a date with her, reminds me of Prime's entitlement issues.
Lois noticed that he eats like a kid.
At first glace this cover is depicting Superman removing his disguise, and that the bloody S symbol is a reference to DOS because tragedy strikes the Kent/Smith family.
I think it's the other Clark Kent removing his shirt and exposing the S symbol scar that Prime carved into his chest. The lack of the scar Connor gave him is because it would be too much of a giveaway, so we'll just be left to assume it healed somehow.
Prime is known to physically age-up when he absorbs extra energy, and the cover to Action Comics #974 has him emitting blue lightning. Prime can subconsciously alter his physical state, this could explain why he's physically human. Let's not forget that the guy is from Earth-Prime and does have reality warping powers, even if he isn't fully aware of them.
I don't want it to be Prime, because I don't think anyone other Johns has utilized the character well since his return, and I don't trust the current creative team, but he seems the mostly likely suspect.