Stupid question: Does that suggest a possible vector for attack, being able to know which witness is next in line to produce a block?
Probably... but when a witness misses a block it's not a big deal and we just move to the next witness.
It's easier to attack 20 witness nodes compared to thousands of Bitcoin nodes.
But again this is a good enough solution... and even if all 20 of our witnesses got punked we'd just elect 20 new ones temporarily.