I tried to find a notion for もう that combines all of its different meanings into one concept. This is what I came up with:
もう shapes the entire feeling of a sentence by emphasizing change.
"もう来ないでください" (it was okay for the addressee to come before)
"９じ時ですからもう失礼します" (the speaker only has to disturb now)
"あいつは、もうだめだ" (the situation with that person is now hopeless, but has not always been)
I really like this approach, but I have trouble applying this understanding of もう to its usage in phrases such as "もう一度" and "もう一本飲みます". Does anyone have an idea how to combine the meaning of もう in these last two examples with the concept of emphasizing change? (Provided that this way of looking at the word is deemed viable at all.)