Metal Slug is a legacy video game franchise that has appeared numerous times on Nintendo systems. The series hasn't seen a new game in years, but it remains popular among a now-adult generation of gamers, and the Steam releases of the original titles have proven massively successful. Parent company SNK doesn't have representation in Smash Bros. despite having a good working relationship with Nintendo (which includes developing games for the Switch); if they were contacted for Smash Bros., I think either Metal Slug or The King of Fighters would be chosen for representation.
On the other hand, Metal Slug is currently not a very relevant franchise, even if SNK has expressed interest in developing a new entry in the series. Furthermore, the series has (pixelated) blood and gore, and the characters wield military weaponry. While it's wildly exaggerated, it could still prove too realistic for Smash Bros.
As for which Metal Slug character would be introduced as playable, it only makes sense for it to be series protagonist Marco Rossi.
Character Name: Marco
Series Icon: Stylized Metal Slug tank in profile
Marco is a short guy who stands with his knees bent, making him about Mario's size. He's fast on the ground and has high, somewhat floaty jumps. He's among the game's lightest characters, on par with Mr. Game & Watch. Marco's crouching animation has him lying prone on the ground; like Snake, he can crawl. Marco's idle animation is the same as the sprite shown above; however, he holds a handgun instead of the shotgun pictured.
Marco is a projectile-based zoning character with powerful special attacks. Some of his neutral attacks also function as projectiles, and he can keep the pressure on enemies easily. However, his projectile-based special moves have limited ammo, which needs to be replenished at random by using a different, time-consuming special move. He's also easy to launch. Marco has to be played as an offensive zoner looking to KO foes before he completely runs out of options, especially since the effectiveness of his recovery also depends on ammo.
Marco's Neutral, Up, and Side-Special moves consume ammo, of which he has a limited amount as displayed by a counter on his stock icon. This ammo can be replenished by using Down-Special. To compensate, Marco's special moves are powerful and come out fast.
-- Alternate colors based on Tarma
-- Alternate colors based on Clark
-- Alternate colors based on Ralf
-- Alternate colors based on Trevor
-- An alternate skin based on his zombie form!
Marco drops onto the stage on a parachute. Upon landing, he takes out his handgun and assumes his idle stance.
-- Jab: Fires his handgun straight forward. Like Mega Man's pea shooter, it's a short-range projectile that mildly flinches opponents. Can be fired indefinitely.
-- Dash: He doesn't really have a dash attack, because he can also fire his handgun while running. The shots have the same properties but slightly shorter range than the jab.
-- Get-up: Swings his combat knife horizontally to his left and right in quick arcs.
-- Ledge Recovery: Jumps onto the stage and swings his combat knife horizontally in a low arc.
-- Up-Tilt: Fires three quick shots from handgun diagonally upward. Good for juggling.
-- Forward-Tilt: Swings combat knife vertically in front of him. Decent range and comes out fairly quickly.
-- Down-Tilt: Fires handgun straight forward while crouching. Same properties as usual, and can also be fired indefinitely. Marco cannot fire while crawling.
-- Up-Smash: Marco crouches, and a boxing glove on a spring shoots upward, out of his backpack. Good range and damage.
-- Forward-Smash: Marco swings the Zantetsu Blade vertically in front of him, which shoots out a short-range vertical energy wave. The sword is much stronger than the wave.
-- Down-Smash: While the attack is charged, Marco suddenly becomes fat. When released, he hops up and immediately drops on his butt, causing a small shockwave. High damage.
-- Neutral-Air: Marco shoots his handgun in front of him. Has the same properties as on the ground, and can also be fired indefinitely.
-- Up-Air: Fires directly upward with his handgun. You know how it goes.
-- Forward-Air: Pokes forward with a trident. Decent range and good damage.
-- Back-Air: The spring-loaded boxing glove in his backpack shoots out behind him. Marco looks back, surprised by this malfunction.
-- Down-Air: As with his down-smash, Marco suddenly becomes fat and drops straight down, but this time holding a large fork to skewer targets with. Good damage.
Grabbing & Throwing:
-- Grab: Grabs the enemy by the collar.
-- Pummel: Slashes with combat knife.
-- Up-Throw: Tosses upward, then strikes with spring-loaded boxing glove.
-- Forward-Throw: Performs a series of lightning-fast punches before sending the enemy flying. Reference to Ralf's Vulcan Punch.
-- Back-Throw: Performs a suplex, bouncing the foe off the floor before sending them flying. Reference to Clark's Argentine Back-breaker.
-- Down-Throw: Places the enemy on the floor and performs an elbow drop, bouncing them off and sending them flying.
Neutral-Special -- Grenade: Marco throws a grenade, which travels in a short arc in front of him, and bounces off the ground once. It explodes upon contact with anyone. These come out fast, and deal good damage and knockback. After running out of grenades, this attack becomes a series of quick combat knife swings that can juggle opponents.
Up-Special -- Shotgun: Marco fires a shotgun directly downwards, which expels a wide cloud of buckshot and sends him flying upwards in an arc. The attack itself deals high damage and knockback, but has a slight wind-up. After running out of ammo, this move turns into a very short air-jump that makes for lackluster recovery.
Side-Special -- Rocket Launcher: Marco fires a rocket launcher that travels somewhat slowly but has mild homing properties, similar to Samus's Missile. Explodes upon contact, creating a large dust cloud, and dealing high damage and knockback. After running out of ammo, this attack turns into a roundhouse kick.
Down-Special -- P.O.W: A rescued prisoner of war appears right behind Marco, and whips out a weapon icon from his pants while grinning. Picking up this icon restores Marco's grenades, shotgun ammo, or rockets, depending on the icon. The P.O.W. then salutes and disappears. If the P.O.W. is attacked before performing his animation, the weapon icon won't drop. Sometimes a special P.O.W. will appear, who will stick around after performing his duty and fire energy blasts periodically. This P.O.W. can be dismissed by inputting Down-Special again.
Final Smash: Super Vehicle-001 Metal Slug
Marco hops into a Metal Slug and fires blue lasers which shoot out in all directions as well as bombs that bounce along the ground. After a few seconds the tank begins flashing and Marco jumps out; the Metal Slug then rolls forward quickly before exploding in a lethal, fiery blast.
-- This seems even more unlikely, but if they were to bring in an Echo Fighter, Fio is a perfect choice and a fan-favorite. She could use a hatchet instead of Marco's combat knife, toss fire grenades instead of regular ones, and her missiles could be stronger but bereft of homing properties.
-- The stage could be based on Metal Slug's iconic stage 1, a beach with ruins and plane wreckage in the background. Enemy soldiers could appear from the edges of the screen in droves, and breaking parts of the scenery could liberate P.O.W's who drop items. Allen O'Neil, a recurring series villain wielding a minigun, could be the stage boss.
-- A fitting Assist Trophy could be the Mars People, cute, octopus-like aliens who glide along the ground and shoot rayguns, or hover in little flying saucers.
The Peregrine Falcon Support Unit: