I shall place input, no?
Well DS being the final boss could happen, though his character doesn't strike me with that role.
He seems to have his own sub plot that is hidden among the stories of TAV.
And so towards the end of the game, maybe those inferences would create a backstory for him and a reason for him to fight TAV.
Also, it seems that each scenario wouldn't have the same ending, so final bosses would vary.
My guess is...
Ven fights MX
Aqua battles DS
Terra battles MX, once Ven losses and would succmb to his darkness to protect his friends
This would make sense to me.
Well DS being the final boss could happen, though his character doesn't strike me with that role.
He seems to have his own sub plot that is hidden among the stories of TAV.
And so towards the end of the game, maybe those inferences would create a backstory for him and a reason for him to fight TAV.
Also, it seems that each scenario wouldn't have the same ending, so final bosses would vary.
My guess is...
Ven fights MX
Aqua battles DS
Terra battles MX, once Ven losses and would succmb to his darkness to protect his friends
This would make sense to me.