In a letter I wrote to Microprose, I asked about a sequal to Rex Nebular, based on some information in the interview with Matt Gruson.

To: Support@microprose.com

I am curious as to why Rex Nebular 2 was never made.  How far into
production did it go?  Why was production halted?  Was money the only
factor in this?  If possible, I would like to see Microprose follow up on
the totally awesome Rex Nebular and the Cosmic Gender Bender with a
sequel.  Thank you for you time.
 

The reply I got:

Hi,

We thank you for your interest in our MicroProse products.  At this time
MicroProse has not announced a new update, re-release or sequel to Rex
Nebular.  You may want to check our web site for new and upcoming titles
from MicroProse at WWW.MICROPROSE.COM.
 

Regards,

Jose
MicroProse, Inc. Customer Support
 

That's all folks.
Sorry, maybe some other company will buy the rights to the game and continue the series.