Senna Runes

So here are some of my thoughts on what Senna should build.

As far as runes, I think she has a lot of options.

First, Keystones

Press The Attack

You might be thinking "wait a minute, she attacks really slow, wouldn't PtA be bad on her?" and normally, you'd be right. However, her Q counts as both an autoattack and an autoattack reset. That's right. AA-Q-AA sets up PtA in a little over a second at 600 range at level 1, dealing a huge chunk of damage in the process. Definitely take this on her. Probably her best rune.

Fleet Footwork

A more defensive option. Similar to Jhin and Caitlyn, her heavy-hitting autos and hefty range make her favor short trades, so the extra movement speed for getting out of those short trades along with the sustain make this a great rune when you can't afford to buckle down and trade or risk getting all-in'd.

Dark Harvest

Improves the consistency of her cross-map "kill secures" and doubles down on the infinite scaling she has going on. Definitely worth considering.

Hail of Blades

She likes short trades, and she likes autoattacking. Three quick autos lets her hammer in a ton of damage, collect some mist, and get out, and do that trade pattern over and over again. Conveniently, it has the same cooldown as her passive.

Electrocute

Similar to Press the Attack in that she can quickly proc it, and the damage she gets is substantial with all the free AD she gets. However, if burst is what you're going for, I think Press the Attack still wins due to the shorter cooldown and damage amp.

Summon Aery

Guaranteed damage/shields whenever you need it. A standard support rune, and the fact that Senna both heals and pokes makes it a great choice for her. It also adds that little extra punch to her snipes if you need it.

Arcane Comet

Her poke is insane, which is then amplified by Comet. She doesn't have any instant CC, so landing the comet can be tricky. Might be worth, but Aery is probably better.

Kaleidostone

I think this one will be sleeper OP on Senna. She can easily proc virtually any rune easily due to her AA-Q-AA combo, her extreme range, and root for Aftershock. The value she gets from chaining keystones together combined with the added early game safety net the other Inspiration runes provide could be hell to go up against. Depends on how balancing it turns out.

For minor runes, you really want to find a way to solve her mana issues. She has a horrendously low mana pool, so even though her cooldowns are long, she can still find herself running out of mana. I think she'll likely need to dip into either Precision for Presence of Mind, Sorcery for Manaflow Band, or Inspiration for Biscuits, especially if you're playing her as ADC. Support Senna will have more breathing room due to the mana regen on support items, but it will probably still be something she wants.