'Must' here is rather BE or formal or dated to my ears, which fits with the Wilde quote. Otherwise Mike's examples all work for me too, and there're lots more variations where those came from. To my ears, especially the more tentative ones have a sort of 'gradually' feel about them that's not unlike 'so langsam' in German.
You know, I should probably be getting ready to go.
I really ought to be going pretty soon.
Oops, look at the time, I really have to get moving now.
Aack, I'll miss my train, gotta go!
There are other cases where the progressive expresses the idea of getting ready, or an attitude of something less than certainty.
Well, bye. I'll be seeing you around. (= tentative, maybe more than once)
I'll be seeing you (in all the old familiar places ...) (= in my mind; I'll be thinking of you)
The line's still busy. He must be having a long conversation. (= probable, happening right now)
Their plane ought to be touching down about now. We should be seeing it taxi in in a few minutes. (= probable, happening soon)
There are also some similar BE/IrishE regional/colloquial turns of phrase that I'll leave it to the natives to explain, e.g.,
I just can't be doing with that.
Will you be going along to the pub then?
We can't be having that now, can we?
It must be getting on for one o'clock.
You'll be wanting to see them.