What would the consequences be if I did (whatever you just mentioned)? Implies that the speaker either believes they are right to do something, or else they are suspicious of the other person questioning them about it. A: "Your team isn't actually going to present that plan to the board, are you?" B: "Supposing we do? It may be a little unconventional, but they'll see that we are right." A: "So, what, you're going to just hop on a plane to Europe and show up on her front door?" B: "Supposing I do? I've always wanted to go to Europe, and at least then I'll know for sure how she feels."