If your aptitude is set to exit when the last view is closed (by default it is), then this can sort of be done already.
When you have an operation in progress that you want aptitude to terminate after finishing, use F6/F7 to switch to every other view and push 'q' to close it. If the only remaining view is the operation in progress (download/update/whatnot), aptitude should terminate after that operation is finished. However, it will rebuild the cache before terminating, which is the time-consuming operation Günter Milde was trying to avoid...
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