A while back Flying Omelette found a bunch of unused stuff while looking through the Blaster Master ROM. This is the original topic, but she said I could compile it into a page on my site, so here it is. FO took the pictures but I drew the colored borders around the stuff so it's clearer what we're talking about.
In the red border: This is the same shape as the cockpit of the tank on the left. Maybe it's the interior, when the hatch is open?
In the green border: These looks like alternate turrets for SOPHIA, like it was originally planned for it to change when you got the stronger guns or changed secondary weapons. There's also a drill in there. Maybe that was how you originally got through the distructible blocks instead of the Crusher, which would later be implemented in Overdrive.
There's also the unused "SPIKE" powerup, which presumably made the tank immune to spikes. This would also be revived in Overdrive.
Is that E-capsule found anywhere in the game?
Little shrinking SOPHIAs. Flying Omelette also found they weren't in the Meta Fight ROM.
Unsused enemies. Also, in the green box with the Maoi head, there's more frames of animation to SOPHIA's conversion to the dive wheels (Deathamster actually caught this one).
More unused enemies (ignore those guys in the upper left of the left picture, they're everywhere, I just couldn't be bothered to draw around them) but these ones, or at least modified versions of them, seem to come back in Enemy Below. The tower is definitely used as the Engine upgrade.
I admit the ones on the left and right are a bit of a stretch, but I think you can see what I'm getting on the left enemy and the one in the middle is definitely a modified version of the enemy with the side-mounted laser - just take off the chunk on the bottom and laser. The right-most enemy is actually lacking a front-facing animation (it can fire up and down, but it still opens from the side), but it seems to be at least based on that opening turret above.
This guy actually isn't unused, but I felt he deserved a mention because he's very reclusive. He shows up in at least one Area 6 dungeon, but people who only go to the boss dungeons (like me) won't see him. His name is "Mudanga" if that's important to you. I think these are also the guys who bum rush you in a couple hallways in Enemy Below, but those guys fly by so fast it's hard to see what they look like.
It looks like Skelevenom was originally supposed to spit giant spiked drills at you along with the blobs.
There's two types of unused lasers here - upper left in the red box, and in the green box. The ones in the upper right of the red seem to match the ones used in the game:
There's also an unused frame of the Boss with his left palm out. Flying Omelette speculated that "he would have probably shot the laser from that hand. That was probably the whole point of him losing his shield in the fight - that would originally send him into phase 2, but instead, he just stays in phase 1, only without a shield."
She also said "the existence of the Spike upgrade in the ROM suggests a 9th stage was probably intended for the game, but when it was axed and the Area 9 boss was placed in succession after the Area 8 boss, their attack patterns may have been dumbed down to make the double-whammy easier to contend with."