Here's my list of assist trophies. These are mostly arbitrary. I basically put together a list of characters that didn't make it in, and narrowed them down to a handful I thought could bring something distinct to the table if summoned. They are:
a.
Biddybugs: a line of them will appear and fly
back and forth across the stage for a while. They won’t do any damage, they’ll
just get in your way.
b.
Bird: It will fly up to one side of the stage,
release its piercing cry and swoop across the stage to the opposite side,
mimicking its Super Mario Land 2 battle.
c.
Bullet Bill: Bullet Bills will fire regularly
from both sides of the stage for the next twenty seconds or so.
d.
Bully: it will come out and do what it does
best, charging at the first person it sees and slamming its full weight into
them.
e.
Coin Coffer: it will randomly wander the stage,
coins popping up behind it. Anyone who manages to KO it will receive a large
bonus in their points at the end of the match. Coin matches I’m sure you can
figure out.
f.
Hellen Gravely: she will pose in the background
and send her cat to do her dirty work. The cat will roam the stage and attack
various fighters.
g.
Huckit Crab: the crab will randomly wander the
stage for a little while, picking up rocks and chucking them at players for
light damage.
h.
King Totomesu: he will stand on one side of the
stage and spit fireballs across it, jumping occasionally.
i.
Lava Bubble: it will pop up at random spots,
leaping out of the stage floor and back down just like in the games. A fiery
lava effect from the games will appear a split second before it leaps as a
warning to players.
j.
Magmaargh: it will lurk in the background just
behind the stage for a few seconds before lunging and leaping over the stage,
severely damaging any fighter it connects with. This will repeat a few times.
k.
Penguin: it will waddle out and then slide on
its belly across the stage a few times, tripping up anyone it bumps into.
l.
Pidgit: it will fly up and cross back and forth
across the stage, acting as an extra platform.
m.
Polterpup: he will sniff around the edges of the
stage, finding hidden items and delivering them back to his summoner for use.
n.
Rocky Wrench: at random spots, his manhole will
appear, rattle for a split second, before he pops up and tosses a wrench. He
will then tuck back in and his manhole will appear in a different spot.
o.
Spawny: he will glitch and fire off a few blasts
of his warping beam in random directions. Anyone hit by the beam will have
their controls scrambled for a little while.
p.
Scuttlebug: it will dangle from the top of the
screen, hanging around until it is collided with, at which point it will drop
and attack the nearest fighter.
q.
Swoop: it will flutter up and hang from the
bottom of a platform. At random intervals, it will take off and divebomb a
character, doing light damage.
r.
T-Rex: it will let out its stunning roar and
then rampage across the stage, similar to the Rathalos assist trophy.
s.
Viruses: they will latch themselves onto nearby
opponents. The opponents will soon take on symptoms of the different ailments
the viruses represent.
t. Wingo: he will teleport himself to the background and flap his wings hard enough to whip up a hurricane, blowing anyone with high enough damage off the stage to faceplant on the screen.
You could probably come up with others. For instance, I forgot about Fuzzies when I first made this, but everything I could think of them doing was basically already done by the Viruses and the Biddybugs
