I've been goofing off lately, sort of picking away at this halfheartedly, but progress is being made! I've implemented some new enemy and hazard types (and refined some of the existing enemies' behaviours), designed and mostly-balanced some new maps, and played around with some of the player's settings. Previously, the delay between shots was limited only by how fast the player could push the 'fire' key, and it was making things a bit too easy on the combat front. Since there was essentially no limit on firing speed, it was possible to just spam bullets and kill most enemies before they got anywhere near you. I've now limited the shooting by allowing only four bullets on screen at any given time. I've played games that use this system effectively, but I'm not really sure how I feel about it. I may try to come up with some other way of limiting the player's power, because the original system is definitely not balanced.
I've been working on sprites for a new area. Here's a WIP screen of the 'terraformed surface' tile set. Also, revamped HUD:

The new HUD still shows health on the left hand side, but now displays score in the middle (obviously), and weapon info to the right. The four arrows will light up in different ways depending on your weapon, indicating charge power for the charge gun, risk of over heating for the automatic, and number of shots available (as per the four bullet limit) for the standard gun, and whatever else I come up with. The olive, tumory-thing is a hive that spawns those tiny green guys, which travel in a wave pattern (like Castlevania's Medusa heads).
Rek: I see what you're saying and I agree, mostly. This definitely doesn't maintain the obviously tiled look of the earlier screens. I don't know how I feel about this, now that you mention it. Generally, I like this look better. The overly-blocky look makes some of the maps pretty visually uninteresting, but at the same time the ultra retro look
is kind of the point. I'm playing around with it, trying to find a happy medium. I tried to keep this in mind when designing the new tile set.
happymonster:I spent a lot of time reworking the sprite in question, and I think I managed to come up with something that's... suggestive of the concept, but still fits with everything else. It wasn't super easy, or fun haha.