There has been progress! Thanks to Zrrion and Kinsie, I have sprite sets for the Grenade Launcher and Lightning Gun, and have been working on them. I put together a video to show off some of it.
A lot of this is still very much work in progress. The Lightning Gun doesn't look or work quite how I want just yet, and the Siphon Grenade still looks a little janky. I'm pretty happy with the Grenade Launcher, barring some tweaking.