New Silas Ult
Given that Silas has been revealed and he's just about to be released there's no way this will be an immediate change or even a change at all. Despite that, I was thinking about how Silas using other people's ults is kind of weird and as someone else mentioned feels kinda weird because how is Silas meant to incorporate an Alistar ultimate into his combo? It would just feel out of place.
Instead, have his ult be something along the lines of: everytime 20 spells are cast near him Silas gets a charge of his ulti (up to 3). To use his ulti is to imbue his chains with energy and swing them around him dealing damage with the charges having a 10 second cd or something.
It fits with the idea of him absorbing energy and then using it again. Dunno how good of an idea this is but just an idea