What are the engine-out implications of giant turbofans?
Both-engine-out glide performance isn't accounted for and isn't a certification requirement (of any airplane that I'm aware of).  Airliners have good glide performance as a happy side effect of design optimization for cruising at high altitudes.
What does have to be accounted for is single engine performance issues; things like departure performance with an engine failure at V1, single engine ceilings, etc.  Gliding ability comes into it in a secondary way, in terms of what is called &quot;driftdown&quot;.  Driftdown is when you are forced to descend from a both engines cruising altitude to the single engine ceiling following an engine failure or shutdown when your power is cut in half, and  could be thought of as &quot;semi-gliding&quot; or &quot;power assisted&quot; gliding performance.
Driftdown speeds are speeds to use to get the flattest descent with maximum continuous thrust on the good engine, and you may use these speeds where terrain clearance means you need to stay at high as possible for as long as possible. Driftdown performance is measured during certification for the purpose of providing data for the pilots in the QRH, but there is no minimum driftdown &quot;flatness&quot; or glide ratio requirement per se.
As far as designing around geared TFs, well, the effects are what they are, and since nobody makes feathering fan blades, you are stuck with whatever the drag penalty is. A windmilling GTF is going to have more windmilling drag than a direct drive TF. But a direct drive TF has more windmilling drag than a traditional turbojet.  So what did the manufacturers do when TFs came out?
They made allowance for the differences, and maybe an airliner would need somewhat more thrust from a given engine in TF form, all else being equal, to accommodate the higher windmilling drag of a failed engine on the other side, compared to the same airplane with pure turbojets. The drag penalties are not as bad as windmilling propellers, and don't necessitate radical solutions like feathering fan blades.
Geared TFs just take the same problem a little farther, but still get away without needing things like feathering fans (which would have killed the entire GTF concept) by making sure the thrust reserve is there to deal with a windmilling fan.  But if a GTF equipped airplane ends up with a both-engines-out glide ratio of 12:1 instead or 14:1 because of the drag of its super-size fans, that's neither here nor there.