If I were to decide the Assist Trophies, it'd consist of...
1. Nintendog (with many variations - Shiba Inu, Dalmatian, Chihuahua, Dachshund, Labrador, etc.)
2. Samurai Goroh
3. Hammer Bros.
4. Tingle (floats into the air and flings neutral bombs - that is, they damage everyone they hit)
5. Professor Kawashima (no ****ing idea what he'd do)
6. Raving Rabbids (running around the arena at high speeds waving tennis rackets and kicking soccer balls)
7. Navi (would annoy the Hell out of everyone)
8. Goomba (minor distraction)
9. Octorok (shoots rocks)
10. Gibdo (does heavy damage to anyone - if damaged, will turn into a ReDead)
11. Koopa Troopa (same as before)
12. Like-Like (eats anyone it comes close to)
13. Waddle Dee (varies - some carry a hammer, some carry a parasol, some carry a sword)
14. Metroid (steals energy from opponents and gives it to you)
15. Karate Guy (from Rhythm Tengoku - punches objects towards opponents)
16. Punch-Out!! boxer (varies - could be Glass Joe, Bald Bull, King Hippo, or Mr. Dream)
Such an awesome feature. Thanks HAL!