Video Game Animation Study recently did a video on Metal Slug 001's animation. Knowing that it was the original main character, it makes sense the animation was so good (even relative to other already beautiful artwork in the game). I spoke with a buddy about this, and he thought it would have been cool if you were the tank, but after you got your armor busted you'd just out as a guy - a la Speed Rumbler, Blaster Master, Boogie Wings, etc.
I'm curious to revisit Slug and see what ideas may have been added after the decision was made to play as a human instead of a tank - how that changed the level design. Even decisions like whether to have diagonal shooting may have been influenced by whatever feeling they were anchored to with the original tank game.
^Would be an awesome proto to unearth.
The aforementioned animation video: