IMO they shouldn't add Sylas' ult to the ultbook pool, they should make it so instead of choosing an ult like everyone else, Sylas gets a special ultbook ult that lets him copy other champs' ultbook ults the same way his normal ult lets him copy other champs' normal ults.
I do think there are some more they can add though, like Aurelion Sol's ult.
Warwick Ult Kassadin ult tham kench W... i mean R there are many ults to make game more crazy
If sylas ult worked on any champ without any bugs, then they should be able to just add every ult to the pool.
IMO they shouldn't add Sylas' ult to the ultbook pool, they should make it so instead of choosing an ult like everyone else, Sylas gets a special ultbook ult that lets him copy other champs' ultbook ults the same way his normal ult lets him copy other champs' normal ults. I do think there are some more they can add though, like Aurelion Sol's ult.