Here's the latest on new controller options for the Atari 5200:
Maker-Matrix has made a new 5200 analog stick gamepad:
https://youtu.be/klmlm05bufo
Ebay seller, Ikonsgr74, has begun making a Atari/Sega 9 pin to Atari 5200 15 pin controller adapter for the 5200 (which is essentially a "Masterplay Clone"):
https://youtu.be/pRCPf85_SYM
There's a new way to get a Commodore 64 mouse working on the Atari 5200 using a Masterplay Clone (or the Ikonsgr74 adapter):
https://youtu.be/y2hsoniR6Bk
Enjoy