Got a new (to me) VPI Aries for Xmas... what’s the arm upgrade path?

Congrats on a fantastic table. I started out with an Aries 1 and gradually upgraded everything but the plinth. The JMW 10.5 is probably a better bang for the buck than the 3D IMHO. I never fell in love with the 3D. On the other hand the Fatboy was a huge step up that made it hard for me (almost impossible) to go back to the earlier ones. But I think tooblue has it right - the one you have is good and the choice of upgrades depends a lot on the other stuff in your system (you may have bigger bottlenecks elsewhere)

You'll want to get the gimbal arm version of the FatBoy. It is a drop in replacement I believe.  That will improve your bass quite a bit. Unipivot arms are bad news on a number of fronts. Manufacturers like Lyra specifically recommend against them. Makers like Graham and Basis go through all kinds of shenanigans' to stabilize them. The Basis arms are not even unipivots any more. They are bipivots. 

IMO a good, and not very expensive, upgrade is the dual pivot.  Do some research.  Besides other advantages, it makes setting azimuth very easy.

+1 @melm 

Before you make ANY changes, live and get to know what the current configuration sounds like. Later on, you will be able to discern what aspects of the table’s performance you would like to improve upon.