Big second on Bill Newton. At BronyCAN (his first) he had a ton of fun, contributed to several panels, and just overall enjoyed himself immensely and was quite sad that was the last BronyCAN. I suspect he'd love to be invited to another convention. Now, if we could manage to get Felicia Day too, that would be beyond phenomenal.
As for Weird Al, if I recall they tried that in 2014 or 2015 and his appearance fee was... on a different level entirely from the VAs, heh. I wish some guests that want to come would sneak over and come on their own if their appearance fees were too high, but they're usually far too busy with their work to take off for a con AND have agents to pay, so it's quite understandable that we'll probably never get them. The hectic life of a celebrity would probably crush the average person x.x