If you bring Greg back, the obvious thing to do would be to have him pay for his sins and treat him like the slimy creep he is, avenging Tanya and restoring justice. But that would be too easy, and not all that interesting.

In S2, we learn that Greg had yet another marriage before Tanya, that she was unaware of. We also learn that he dropped acid back in the day with Quentin. And even though he clearly is a horrible person for executing his con, we don't know all the specifics of it and we know that he at least was trying to give Tanya a good time and let her live her best life before her death, hinting at something of a sense of guilt and remorse for doing what he did (obviously not enough to deter him from going through with it though).

There was a line in S1 where Tanya recounts a dream where she was "on top of this beautiful mountain range in Asia and I had a cyanide pill”.

We now know that S3 will be set in Asia and deal with Eastern spirituality and life and death.

The through-line across two seasons of the White Lotus has been Tanya's wealth, so why not conclude that arc in the third?

What if there is more to Greg than we know. What if he has kids from other marriages that he is hellbent on leaving behind money for, in a Walter White kind of way, especially considering the cancer diagnosis. What if he has this other side to him where he was kind of a hippie dropping acid back in the day, doesn't really agree with excessive wealth and shallow, privileged lifestyles like Tanya, and he made a conscious decision to take Tanya's wealth, but do something good with it. Whether that means leaving it behind for his "real" family, or some to charity, or whatever, his long con on Tanya was always going to end like this, transferring wealth from someone he believes doesn't deserve it, to ultimately serve a better purpose.

So after he secures Tanya's wealth, and updates his will so that all the money is left behind how he wants it to be, he goes to a White Lotus resort in Asia, tries to find some type of peace with himself (ayuahasca trip like his old acid days?), confront the sins he committed against Tanya, and then take a cyanide pill on top of a mountain in Asia, committing suicide and completing the last step of his long con.