I think that they should add this feature to the wep, since the Engi vs Spy update is coming soon.
When you open the umbrella, with alt fire, and you point it at the sentry, it will act like a wall, so sentries won't be able to see you.
Also, being able to open it and close it everytime like with the BASE jumper would be op, so it will have a timer like the ball of the sandman.
This could be useful if you are using the ethernal reward and you see a sentry and there is nobody watching it.
They should also add a defensive buff. If you open it, it will block something like 100 dmg or less, and then it will break, like the razorback or just expire like the spycicle for a long time.