Of the three options you suggested i like "Hourglass Not Done" (⏳)
best, because it most clearly shows that we're waiting for something.

Or how about Stop Sign, 🛑?

Do you want `ent list` to show the number of issues that this issue is
blocked waiting on?  Something like:

    012345       This issues stands alone
    abc123  ⏳4  This other issue depends on four things