Well, here's a few ideas.
The previous poster, Richard, says "I ignore silver recover....my output does not warrent the trouble." I think this is a pretty common view on this site. A few years back, one of the members did a survey to the effect of "what do you do with your used fixer?" I can't find the survey, but as I recall, the majority reported doing nothing to recover silver. It is my opinion that most of them will have NO IDEA of how much silver they are handling. So some way for the typical user to discover these quantities could be useful.
You mentioned darkroom water supply. Nowdays, most municipalites have (typical) water analyses available online, but probably most photographers don't know what to look for. It would be nice if there was a simple method for them to know if their water was ok for 1) mixing commercial developers, 2) mixing handmade developers with no sequestering agents, etc., or 3) for washing. It would be nice if there could be (cheap) test kits for various things, OR perhaps even a (cheap) analytical service where they simply mail a small plastic bottle of water to a test lab, and get back a report on "suitability for photographic use."
Regarding darkroom ventilation: I think most people have no idea what to do about this. It might be useful to have an on-line calculator where they could put in their darkroom dimensions, and perhaps layout, as well as the inlet air and possible exhaust locations. From this, various exhaust specs, or even specific blower suggestions could be made. Or, perhaps an inexpensive photo chemical "mixing hood" with integral blower and filter could be designed; just connect an exhaust duct. Such a device might be able to also serve as the darkroom's exhaust fan.
Hope this gives you a start. I have no idea if there is any longer enough darkroom-user market for any of these.