Sure, it could be Harry - but only for one reason - Vold. caused Harry to be an orphan. Neville, through the actions of death-eaters led by Vold. - was caused to be an orphan as well. (More or less). Also, and this is KEY!! It doesn't say WHEN Vold. will mark "him" as an equal!!!!
I think it is harry, 4 a couple of reasons. first of all, he DID recieve some of voldemorts powers. second he feels his emotions, and third, voldemort, in the fourth book, wanted harrys blood NOT nevilles.
and even tho, harrys being a half blood had NOTHING 2 do with the fact that voldemort (marked?) him, it is still interesting that he felt (if he did mark harry) that harry was more threatening. especially when we kno that nevilles parents were highly respected aurors and he was a pure blood, there4 "better" than a mudblood (like harry)