Turn acceleration is going in at some point. Probably after a menu clean-up. (There's a toggle for fast versus smooth rendering, but it's hidden below the last visible option.) In the meantime... Mesen lets you plug an SNES mouse into the second port.
Portals were another weirdly easy absurd feature thanks to raycasting. If you get a close look at enemies through one, you can tell the math is a little goofy. The surface was originally supposed to wobble or flex continuously. That's now limited to when they're shot, because the feedback is hard to distinguish at these framerates, Static-by-default lets one tucked into a corner look like contiguous space - like the one in that high alcove. (Which you can get to, with the glide-y jumping. Definitely keeping the bizarre platforming.)
The shotgun and machinegun might get combined. An amalgamated pew-pew weapon that fires differently based on how you hold B is extremely well-precedented on NES. Though it'd act more like you're always charging a shotgun blast, press B to release it, and spit bullets continuously while holding it. That's honestly more likely to go in "Slaughter 2," after this is complete enough to call a proper game. Anyway that'd free up Select to immediately fire rockets, making them easier and more useful. But so would nerfing the current rate-of-fire so bullets and pellets can get punchier. Even as a firehose of bullets, the sound effects make them sound like a polite suggestion. There'd be some power-up that brings back this rate-of-fire and absolutely rips through enemies.
Enemies should also be crammed into every inch of this level. Blue zombie guys in the green underground. Rabbit tank things around the exit. Their distribution hasn't changed since the compo, because I want the data stored in the same compressed format as the passwords, and that format is currently thrown out so I can start over. (This was not aided by finally discovering that reading data back from VRAM storage begins with a byte of junk. Nintendo. Whyyy.)
All of which has to wait until after GB Compo, which runs for another month. This update was an excuse to avoid my initial project. Two weeks ago I said "Screw it, I wanna play with scanlines!" and honestly it's been great. Expect 60 Hz parallax and a vulgar bird. Then more pew-pew NES stuff.