Gimmick:
In their games, the Battletoads transform their limbs when they are about to inflict the finishing blow to an enemy. To represent this in Smash, one potential idea is to power up their attacks when the opponent is at 100% damage or higher. This would be visually represented by having their limbs transform. If this wouldn't work, just having their limbs always transform for attacks can also work.
Weak Attacks:
A Combo — Kiss My Fist
The Battletoad attacks with two regular punches followed by a big punch where his fist grows.
Forward Tilt — BT Big Hammer
The Battletoad attacks by swinging both hands left then right. If the opponent is at 100% or higher, the attack will be replaced by a giant hammer.
Up Tilt
A short uppercut
Down Tilt — Ball n’ Chain
The Battletoad delivers a leg sweep. If the opponent is at 100% or higher, this attack becomes stronger and his foot is replaced by a spiked ball and chain.
Running A — Charge n’ Barge
The Battletoad delivers a running head butt to his opponent. If the opponent is at 100% or higher, this attack becomes stronger and the Battletoad has ram horns appear on his head.
Smash Attacks:
Forward Smash — Big Bad Boot
The Battletoad punts his opponent. If the opponent is at 100% or higher, this attack becomes stronger and the Battletoad’s foot turns into a big boot and he jumps in the air a bit.
Up Smash — Stone Fist
The Battletoad delivers a fierce uppercut. Normally, his hand just grows bigger (like many of Mario’s attacks), but if the opponent is at 100% or higher, this attack becomes stronger and his fist turns to stone.
Down Smash
The Battletoad clasps his hands together (like Donkey Kong’s Aerial Forward A) and smashes his opponent into the ground (like Donkey Kong’s Forward B).
Specials:
Neutral B — Walker Leg
The Battletoad takes out a walker leg. After striking with this a few times, it leads to a command grab where they lift their opponent up and can choose to either smack them forward or toss them backward.
Forward B — Speeder Bike
This could work exactly like Wario’s motorcycle, except they would be using their speeder bikes from the Turbo Tunnel stages.
Up B — Rope
The Battletoad swings his arm like Donkey Kong’s windup punch and throws a rope upwards and attaches to any surface that he uses to swing like Spiderman. If you press B while swinging, he can turn into a wrecking ball and attack the opponent. This can be used as a tether recovery.
Down B — Drilla Killa
The Battletoad launches forward with an drill, using animation like Metaknight’s Forward B. When using it in the air, it would travel diagonally downwards like Falcon Kick.
Grabs:
Grab
The Battletoads have a tether grab with their tongue similar to Yoshi. Unlike Yoshi, the toads don’t swallow their opponents and only drags them closer like Link’s hookshot. Like the hookshot, the toads can also use their tongues as another tether recovery.
Pummel
The Battletoad kicks their opponent in the gut.
Forward Throw
The Battletoads pick up and carry their opponent similar to DK and can toss them in several directions.
Back Throw
The Battletoad grabs their opponent’s head and smashes them back and forth into the ground.
Up Throw
A simple toss upwards
Down Throw
The Battletoads jump in the air and slam their opponent down similar to DK's throw.
Aerial Attacks:
Neutral A
The Battletoad attacks with his arms outstretched and hitting enemies on both sides.
Forward A — Anvil Swipe
Something very similar to Gandondorf's. If the opponent is at 100% or higher, this attack becomes stronger and his hand is replaced by an anvil.
Back A — Flying Battleaxe
This attack would look a lot like Fox's. If the opponent is at 100% or higher, the move becomes stronger and his foot could become an axe.
Up A — Soarin’ Saw Blade
The base attack would be a standard somersault kick like Ganondorf. If the opponent is at 100% or higher, the move becomes stronger and his feet are replaced by a Buzz Saw.
Down A — Wun Tun Stomp
The Battletoad stomps down with one foot similar to DK but it causes the Battletoad to drop down like Toon Link’s Aerial Down A. If the opponent is at 100% or higher, this attack becomes stronger and the battletoad’s foot is replaced by a huge weight.
Final Smash
This could play pretty much exactly like Pikachu’s Final Smash. The Battletoad could turn into a spiked wrecking ball instead of an electric one. Afterwards, all of the Battletoad’s attacks can be their fully powered up versions for a little while.