Doesn't have to fit into design, just anything that you think is cool. It doesn't even have to be balanced if you don't want it to.
For example: Ultimate: X reduces the cost of a complete item of choice by 500 gold.
You know what would be ridiculous? A point and click nuke that scales with missing health
Ultimate: Stop time in the area around the champion for X seconds. Allies, enemies, projectiles and over-time effects are frozen. Only the caster champion can move and act within the area. After the time is up, everything goes on the way it would have been before. Over-time effects like (de)buffs, DoTs or AoE fields don't deal damage during that time, but also don't expire. Their duration and effect resume after the Ultimate ended.
Passive: X can only cast abilities in subsequent order. (Similar to Ryze) [Q E repeating and must end with W before going on a 10 second cooldown after 4 seconds of not casting abilities, otherwise it will go on a 25 second cooldown after 6 seconds and you can't use any ability besides your W. If you use your W first before any other ability the passive cooldown will remain at 10 seconds] When X's E is casted twice within 6 seconds it deals triple the base magic damage.
Q: Some zig-zag skillshot ability that speeds up/deals more damage the more minions/players it hits. (Like Brand's Ultimate and Jhin's Q) W: A shield that's sort of a combination of Karma's shield and Urgot's. Speeds you up when you use it, and slows players by 5-10% (Based on Level) who try to attack you. E: X throws an electric projectile (Point and Click Ability Similar to Viktor's Q) that spreads to three other targets and deals 25% of total Ability Power. They are marked and when a Q hits them it deals 120% of the original Q damage and E's cool-down resets. When it's used any additional time within 6 seconds the base damage is stacked up to 6 times (150% of total ability power) R (Toggle): X's cool-down reduction has increased to 50% and the cool-down on X's passive has reset. Any ability casted in the next 2 seconds is manaless, otherwise if not toggled off after 3 seconds, he is stunned for 2 seconds, shockwave doesn't go off, and his armor is reduced to 12 for the next 6 seconds. When the ultimate gets toggled off, X releases a shockwave area of effect ability that deals 300 magic damage with 25% AP scaling that stuns anyone in the surrounding area for 2 seconds including himself for 0.5 seconds, his armor is reduced to 0 for the next 6 seconds but is immune to crowd control effects, excluding suppression. ** I know you said an "ability" but I got a little carried away and made this all up. :>**
Passive: All nearby enemies die every second. (Range: The whole map)
Do true damage that scales with 20% of your AD and does increased damage based on how many times you've auto attacked them or killed someone else recently with this spell as well. If you killed someone with it, it's cooldown resets and at lv 3 it no longer costs mana
on the oce board i put a make a champ thing, i had this one cool ability
summons around champ when cast slows enemy and self projectiles that enter and as the feild is out longer, the slow becomes a complete stop over 3 secs, to even those just entering, then when it ends after 2 secs after the complete stop, all projectiles fire out at double speed with friendly fire enabled (only for enemy projectiles)
Starcall: Stars fall from the sky and hit every target around you in a radius. It deals small amount of damage and shreds MR.
A mirror which absorbs an ability and copies it, recast to use the copied spell within 8 seconds
ultimate: beast tamer
traps a non elite neutral monster and grants the same gold and exp as defeating them and can then release the monster with improved stats based on your attack and defence released monsters prioritizes champions>turrets>minions
Hunting targets as 
evolve points where he can indefinitely upgrade his abilities. same with 
ult dropping daggers on the ground would be awesome but game breaking
Passive: Ddos, raises the MS to 500 of the last unit that hit you, including baron.
Hey, you said it didn't have to be balanced.
point and click 10k true damage on a 4 second cd has 18 passives made by certainlyT
Passive: Killing an enemy champion permanently gives +10 movement speed. Your Attack Speed and Attack Damage both scale with your movement speed.
Kills/assists give 10 ms permanently Your as scales with 30% ms and ad scales 10% ms.
Q: short sword slash deals 60+110% bonus ms+100%ap. shoots in a straight line. Basically, a mini ezreal ult that scales with ms because that is this champs gimmick.
W: Passive: Gains extra gold for killing monsters/minions based on 10% extra ms. Active: Gain gold equivalent to your ms. (very long cooldown, would shut off the passive)
E: gives you 200% ms for 2/3/4/5/6 seconds.
R: Slows everyone within a small area, and you gain 30 ms permanently.
the flash in league. Basically, Master Yi + ezreal +gangplank.
Ultimate: Turret Breaker
A skill that actually can only be cast on enemy turrets. When activated, a turret struck receives 25% max health true damage and reduces the turrets defense by 50% for the next 10 seconds. Timer cooldown is 5/4/3 minutes.
