Now who would play Smaug? Pepe Le Pew?!!

like i said, they wouldnt NEED to substitute a given Looney Tune for every single character; they used one-offs all the time, like throwing in Nero in a Roman setting for instance (which admittedly also had Yosemite Sam as a centurion), or that random Scotsman from when Bugs accidentally tunneled to Scotland while trying to find Los Angeles. the characters would just need to be ersatzes cast in the proper Looney Tunes light. like so:
“Smaug”: Thieves! Fire! Murder! Who took my golden cup?!
Bugs: Ehh, what’s up, drake?
“Smaug”: You! How dare you trespass on my lair?!
Bugs: Who’re you to talk?
“Smaug”: I! Am! SMAUG! My armor is like tenfold shields, my teeth are like—
Bugs: Hold on, big guy. I got an idea. (whispers something into Smaug’s ear)
[[outside]]
“Thorin”: What’s taking that rabbit so long?
Bugs: Hey, boys! Come on in! I got the dragon busy!
“Thorin”: Really? That’s great! Come on!
[[the dwarves run in, only to find Smaug looking right at them]]
Bugs: Now YOU’RE gonna listen to ME.
[[fade, cut to Bugs and Smaug managing a burger joint or something while the dwarves have to advertise for them and clean up]]