My idea for an interesting and simple marksman design
The concept
Someone trained to use wind magic by Janna. Their attacks are the classic "wind cutter" or "air blade" concept and they have wind-themed abilities.
The abilities
Passive - A Thousand Cuts
Every subsequent attack on-hit deals an additional 1% of the target's current health as physical damage. Stacks reset after 3 (+0.5 per level) seconds.
This empowers continuously attacking the same target while not scaling into ridiculous damage. Since it's current health based, it disproportionately provides a benefit when attacking tanks. The shorter duration early makes it easy to drop stacks in lane while allowing for more breathing room in mid/late game team fights.
Q - Wind Tunnel
Connects you and the target with a tunnel of wind that latest for 10 seconds and grants 20/25/30/35/40% attack speed against the target. Being more than 900 units away breaks the tunnel early.
A simple stat check ability that has interesting interactions with the rest of their abilities.
W - Gust
Against a target affected by Wind Tunnel, slows the target by 20/25/30/35/40% for 2 seconds (no decay). Against another target, knocks them back 150 units and slows them for 20/25/30/35/40% decaying over 2 seconds.
When on the offensive (with Wind Tunnel), this provides some kiting and gap closing ability. When on the defensive, the knockback and slow
E - Vortex
For the next 2/2.4/2.8/3.2/3.6 seconds, blocks all projectile basic attacks with a 50% chance of reflecting the attack back to its origin. If a target is affected by Wind Tunnel, this chance is increased to 100% and attacks are reflected to the target of Wind Tunnel. While Vortex is active, enemies within 350 units are slowed by 10/12.5/15/17.5/20% and take 30/40/50/60/70 (+ 15% bonus AD) physical damage per second (0.2s ticks).
R - Wind Blade Barrage
Pushes the target back until they hit a wall or are 1000 units away. If the target is pushed against a wall, they are slowed by 100% (not affected by Slow Resistance). Gain 50% total attack speed (can exceed 2.5 attacks per second), 1000 attack range and self-taunt to the target. Channels for 4 seconds or until the target is pushed to 1000 units away.
This provides both a CC with damage burst (that can still be flashed out of under the right circumstances) or a single-target knockback for a clutch escape.
There's probably room for some more diversity in their abilities (there's two different slows for example) but I like the theme and the simplicity of being a basic attack oriented champion with some utility and defense.