Unfortunately my response was too long, so I put it in a public google doc.
Link: https://docs.google.com/document/d/1k_1QeMfyLui0la4W0pq1thPdpkU5wiKgtJRMHvd2o5Y/...
Just to answer the "clutter" thing that I mentioned; I mainly meant how the stacked items and other HUD elements follow the player, but are more bright than the actual slug, so I'd often find it hard to keep track the slugs center position if I glanced away. I think it's probably just one of those things you'll need to play around with, but I'm sure you'll figure out a better design.