Lightspeed

Rogues have many specializations they can chose from. Some are more thieving, choosing to tap into their cunning to master the art of the steal. Some chose to refine their assassination arts to masterfully execute stealth execution operations. Some even find ways to tap into the deep arcane arts, choosing to devote much time to refining their deep wizardly knowledge to cast and refine spells with their rogue tactics. 
Few chose to specialize in something most Rogues would consider quite as flashy as raw speed. Those that do see speed as what it really can be, a resource for all avenues of Rogue antics. With enough speed, sneak attacks become an easy option. With enough speed, you can be in and out of an area or even a pocket fast enough to not need to rely on pure stealth. With enough speed, one can even accomplish magical feats that others might not be able to predict. 

Level 3: Level 3: Move it Move it

Your raw speed is growing to exceptional levels. At Level 3 you gain 5 ft of max walking speed. Gain 5 ft of max movement again when you reach level 7, 11, 15, and 19. 
In addition, you have advantage on Skill Checks made after moving 20 or more ft in one movement.

Level 3: Level 3: Kinetic Snap

Your speed has begun to generate such a high level of kinetic energy, that you have begun to harness it into your steps. If you have not used your sneak attack when you use a cunning action AND move 20 or more ft, you then exhaust your remaining movement to fling your foot forward and shed your kinetic energy as a bolt of electrical energy. Make a ranged spell attack against a creature within 30 ft of your current position. You are proficient with it, and you add your Dexterity Modifier to its attack and damage rolls. If this attack hits, the creature takes damage equal to your sneak attack as Lightning damage, and you can not use a sneak attack for the remainder of the turn. 

Level 9: Hands of Speed

Your speed not only effects your footwork, it also enhances your arms and hands. You gain proficiency in Slight of Hands. 
You also may make an Unarmed Strike as a Free Action whenever you have moved 20 ft before entering melee range of a target creature. 

Level 9: Turbulent Aim

You know how to keep sight of the enemies while moving. You may use Your Steady Aim feature by only losing 20 Ft of movement.

Level 13: Bolt is Life

Your kinetic lightning has become so strong that it can shock almost anyone at full force. Your Lightning Damage ignores Resistance (no effect on immunity). 

Level 13: Keep Momentum Going

If you move at least 30 ft this turn, you gain +2 AC and advantage on Dex saving throws until the start of your next turn. 

Level 17: Current Conversion

Your Kinetic Energy is so strong it oozes out of you. When you hit with a sneak attack the target creature makes a Dexterity Saving Throw (DC 8 plus Dexterity and Proficiency Bonus). On a fail, the creature has disadvantage on all D20 tests until they move at least 5 FT.