Will we have other Champions in the future with fixed attack speed?
Jhin's fixed attack speed definitely increases his complexity and makes him interesting, but I think we'll need more than one Champion with such a mechanic.
For instance, what about a Yordle in a Tank? Yes, this is the World of Tanks casual in me talking, but just imagine a Yordle in a midget version of a Königstiger (a German Tiger II tank) or piloting a Yordle version of a Terran Siege Tank (Starcraft). That'd fit perfectly for "slow rate of fire, devastating damage per shot".
If Blizzard can make it work with Sgt. Hammer in HOTS, Riot can certainly do it with League.
EDIT: Asking questions seems to make you a downvote magnet.