I think that using the Olympic Games as the reason for Mario and Sonic to crossover was a really cool idea, but it's a shame that the actual games I played from that series were kind of mediocre.
Himself (I don't think they'd reference WarioWare any further than WarioWare, Inc., and Wario is always only in things for himself anyhow)
Waluigi
Himself
Dr. Eggman, Metal Sonic, Egg Pawn
Eggman Empire
Jet, Wave
Babylon
Zavok, Zazz
Lost Hex
Knuckles
Angel Island
Blaze
Sol Dimension
Every unlisted Sonic character would sign under themselves. (There's a chance Eggman Nega would sign under the Eggman Empire, but he hates vanilla Eggman so I feel like he wouldn't.)
(And yes, there would absolutely be a silly story reason for the International Olympic Committee agreeing that the non-existent Eggman Empire counts as a nation. Eggman has his ways.)