What kind of cheat unit would you like to see in Age3? (or perhaps the X-pack) Considering there are only two currently...
My favorite would be the multi-armed archer. It'd be a normal archer, exept with 4 arms on each side, and shoots 4 bows at once. Optionally, it could be muskets instead. And maybe he could be riding a super-fast horse. Or maybe a motorcycle.
Or, maybe some kind of small, hovering rodent, who simply flies towards its victim, latches on, and instantly kills it. (If you've seen Monty Python and the Holy Grail, you'll know what I'm talking about :P)
Also, I like units who are funny, but aren't impossibly more powerful than anything else. Things like the multi-armed archer mentioned above, or maybe a cheat that spawns an army of dogs (all with some obscure name, like "Pete" or something), at your home city gather point. Similar to AoM's "I love teh monkeys!" cheat. (which spawned hundreds of controllable monkeys around your base)
Oh, and something hilariously modern and out of place, like an ambulance or a woman with a taser. Or maybe some random AoM god power you could activate.
"Build a man a fire, and you warm him for a day. Set a man on fire, and you warm him for the rest of his life." -Chinese proverb
"If MacGyver and Jack Bauer were in a fight, Chuck Norris would win."