the ultimate theory

Well the ultimate theory or unified theory or supergravity theory is about a simple theory that explains all phenomena.

As of now things stand at E=Mc2.
And the options ahead as noted by Stephen Hawking are

1. There is a complete unified theory
2. There is no ultimate theory but an infinite sequence of theories such that a particular class of observations can be explained by a particular theory upto a particular finite point.
3.There is no such theory, but observations. Beyond a certain point you cannot predict with sufficient accuracy and so is arbitrary.

I think there is a 4 option largely on account of a limitation.
And it is a mix of #2 and #3.

Heisenbergs Uncertainity Principle makes it pretty clear to us that we can only choose to measure either speed or position accurately, not both.
On account of this very theory, we will always be in a position to explain a phenomena upto a certain finite point, beyond which we will get arbitrary.
We become arbitrary as a result of the fact that we can be accurate of only one speed or position, our inability to be prescise about the other will render some element of arbitrariness.

If we intend to go deeper, we will find another explanation and so on and so forth.

I guess in a sense we are limited always, maybe that was put in when we were formed. Like we know there exists a limitation in everything we build which in a sense implies we were built or rather there is a creator.