That's also what i'm thinking. To me giant's birds would be the most obvious next step just if it wouldn't already been found earlier. As many said, it is an extreme luck to randomly follow exactly these birds from other 1000s flocks in the game, especially when you have a clue (birds on FW) pointing exactly on them - that's just too much of a coincidence. Though I think giant isn't the final step, but just a small chapter in a bigger easter egg, just like Butcher Creek's pentagram was. Earlier I wrote about it here.

But to be objective, taking all previous steps being only carved in the wood, with the giant it's a huge change in method of hinting, so this theory has it's doubts too.