Marksman

A skilled marksman excels at long range skirmishes more so than the typical rogue.

Steady shot

At 3rd level, the marksman no longer suffers disadvantage while firing crossbows while prone. In addition, the marksman has advantage while shooting crossbows from the prone position as long as there are no enemies within 10 feet and their target is visible or otherwise detectable by you. In addition, you gain proficiency with both heavy crossbows and light crossbows.

Crossbow Repeater

By 9th level, you've learned to construct small cartridges loaded with crossbow bolts. You may modify your crossbows to be able to fire a number of bolts equal to your Intelligence modifier + 1 (minimum 2) before having to use your bonus action to reload. The first time you modify a crossbow in this way, it is free, but if you lose or damage your crossbow in any way you must spend 50 gold in relevant supply costs to repair or modify another one. You also gain proficiency with Tinkerer's tools. In addition, you carefully maintain your crossbow bolts. As part of a long rest, you repair or maintain your bolts in such a way that they cannot be destroyed or lost through firing them unless you purposefully fire them somewhere you cannot reach such as in lava or down a bottomless pit. When recovering ammunition after a fight, you recover all your bolts instead of half as long as you spend at least a minute searching for them. 

Sleep Dart

Your time adventuring has enabled you to take out targets at range in a nonlethal manner if you so choose. Beginning at 13th level, you may create a slightly toxic sleeping dart as part of a short rest using a poisoner's kit (you also gain proficiency with the poisoner's kit) and consuming 10 gold's worth of supplies. When hitting a creature with this dart you deal damage as normal. However, if the damage reduces the target to 0, they don't die and instead fall unconscious for 1d4 hours. Creatures that aren't reduced to 0 by this attack instead suffer as if the sleep spell was cast on them using your sneak attack dice instead of the normal die roll.

Memento Mori

At 17th level, your shots fired from a crossbow have gruesome results. Your first shot in a combat against creatures that are surprised is always a critical strike. In addition, if your shot does more damage than half the hitpoints of the creature fired upon, enemies within line of sight and within 60 feet of the target must make a wisdom saving throw against your crossbow shot DC (8 + proficiency + DEX) and on a fail they are stunned for one round. If your shot outright kills the creature targeted, the enemies have disadvantage on the saving throw.