I also played alot of Lissandra in the end of Season 7. I even played her as a viable support, despite the backlash i recieved and the controversy it caused.
And from my experience with playing Lissandra, these are the buffs i would like to see.
Passive - While the passive is avalible, the next ability Lissandra uses will have slightly increased cast range as well as a 0 Mana cost.
Q - Damage against units inflicted with hard CC (root/stun) is doubled.
W - Now interrupts mobility abilitis. Example: Lissandra will root Jax in the air while he is jumping on her, and Lissandra will not take any damage.
E - If Lissandra scores a kill or assist, 50/60/70/80/90% of the ability's cooldown is refunded.
I sincerely believe that her passive is right now nothing more but a Manaflow band, and while my suggestion preserves its original identity, it also adds a new interesting twist to Lissandra's kit.
She and her opponents will now have to keep track of her passive more. They will also have to learn to adapt and play around new ranges, which will prove to be fun and challenging.
Her W change is in order, because Lissandra has always been a very viable pick against melee mobile champions, and this buff will further increase that viability.
The E change is meant to give Lissandra more late game mobility, as well as make her assassin playstyle a bit more viable.
But ofcourse, playtests will best prove if my buff ideas are good. So, i sincerely hope Riot notice this and perhaps implement it on the PBE.
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