In current shape, the upcoming spyglass is, at least in my opinion, not particularly useful device, especially for mod users.
Several mods already come with an option to take a closer look at faraway objects.
Additionally, a vague look from distance already gives enough information to understand the situation in most cases.
In order to change that, here are two of my ideas:
1. Rangefinder built-in every spyglass. It would show the distance in meters/blocks from the first solid object the crosshair is pointing at. It would be of great use for builders in measuring their constructions.
2. Crafting with together with bow.
When pulling, the player would be able not just to find ranges, but the spyglass will have a parabolic correction grid drawn on it. However, the player will have to adjust the parabolic falloff himself, the bow will not correct the movement or adjust elevation on its own. I think it would also be highly educative in terms of learning how a scope works.
The balancing factor could be longer draw-time, or a delay between switching to spyglass and starting arrow draw, decreasing rate of fire, and therefore DPM of the bow. The scope also limits vision, so it would not let the player aim correctly when firing over really long distances, where arcing is to severe to fit in the scope, as well as forcing the player into tunnel vision
Naturally, spyglass would be detachable without any enchantment loss, and used up bow, rather than breaking into nothing, would leave the optics intact.
In recent times Crossbow took over much of Bow niche the crossbow enables two various playstyles depending on ammunition used - shield-piercing, reusable arrows and high-damage, AoE rockets, while bow allows only one (high damage special effect arrows), so the scope will allow a second one to even the odds, but in completely different manner.