As noted, next week's WWE RAW episode will be a RAW Reunion special with a few dozen Legends and Hall of Famers making special appearances. Dave Meltzer noted on Wrestling Observer Radio that the original plan for the next reunion special was to hold it on the first SmackDown on Fox episode, which will take place on Friday, October 4.
There's no word yet on if they still plan on doing a Legends-themed show for the first SmackDown on Fox.
It will be interesting to see if the SmackDown 20th Anniversary special will air on the USA Network or on Fox. The very first episode of SmackDown aired on April 29, 1999, which was a special on UPN. It became a weekly show on UPN on August 26, 1999, so the 20th anniversary of the weekly series would be next month.
There's no word yet on when that show will air, however WWE could have the very first SmackDown on Fox also be the 20th Anniversary special since the SmackDown anniversary specials are usually held in late September or early October. The 5th Anniversary special took place on September 23, 2004, while the Decade of SmackDown special took place on October 2, 2009, and SmackDown 15 took place on October 10, 2014.
There's no word yet on plans for the SmackDown 20th Anniversary special and while they might not hold another full-blown Legends reunion show just months after the RAW Reunion, it's still likely that they will bring back at least some Hall of Famers and Legends for the SmackDown 20 special.