There could very well be a third-incarnation of Xehanort.
When Xemnas was destroyed, his soul could have moved on and, through Xehanort's determination, found a way into being.
As for the DHK and the bald man, well, I'm sure there are more people other than me who recognize the extreme similarity between the DHK and Riku's dark form.
But, allow me to rule out the bald man being Ansem the Wise.
When Mickey showed up at Radient Garden, it was the first Ansem had heard of the keyblades or other worlds.
Also, Caminee, Ansem the Wise always knew it was Riku. That's why he laughs when Riku introduces himself as Ansem. He says that he felt so much grief over being held accountable for Riku's situation that all he could do was laugh. He alays knew it was Riku. Riku had been working with him since the end of CoM.
Ansem 100% despises Xehanort.
Xehanort took his identity, his work, his home, his students, everything!
But all that aside, this is supposed to take place in the past.
Personally, I used to think that the shadowy figure was Xehanort but now... I think they're just lackies.
Xigbar is deffinitely involved somehow.
But I will admit, the bald man IS wearing Xehaort's Heartless' shoulder pads.
And the DHK is wearing a suit similar to Riku's dark side which was only attainable through the dark power Xehanort gave him.
I don't think we'll ever strumble acroos anything that isn't Xehanort involved.