I know with toning I can get various shades of brown on paper. But why can't that be done with the developer and save a step?

Example: could you add selenium or thiourea to a developer formula to turn the exposed silver halides to brown-ish rather than soft or cold shades of grey?
Warm tone developers on warm tone paper are still grey.
could it be done as a second developer before fixing?
I see from the toner thread that using pyro in the redevelopment is an olive tone, could that be done in an initial developer?

Or could the paper manufacturers reinvent the coating process such that the silver is mixed with other metals to change the color?

Just pondering.