Kay, so Senna's shikai is pretty badass in my opinion.
So I suggest this on how to make it implimented in-game!

Shikai
Boosts: 1.3x reiatsu power, 1.5x defense. Movement rate increased.
Attacks:
Hitofuki [that's gust in japanese. damn i'm original.] - A simple 1x1 projectile, becomes 1x3 when the shikai is mastered. Based on reiatsu power. 7 second delay. x1 damage multiplier. Uses 100 reiki to use.
Jiten [spin in japanese. i am on a roll.] - Another simple skill, damages the area surrounding the user in a simple square. 4 second delay.
x = tiles hit
o = player

xxx
xox
xxx

Bankai
In contradiction to the simpler shikai, bankai can be used intelligently! Yay!
Boosts: 1.3x strength, x2 defense, 1.6x reiatsu power
Hiru-azukari [leech + draw] - Double AoE. By that, I mean it hits like this:
xxxxx
xxxxx
xxoxx
xxxxx
xxxxx
Anyways, this doesn't do any health-damage. It pulls everyone inside the radius of it closer to you, and drains reiki. The attack drains 1/20th of the other's reiatsu, and costs 4000 reiki to use. 1 second cooldown.

Fugu Hitooshi [cripple + push] - Double AoE, same range as Hiru-azukari. This technique pushes the opponent away, and temporarily slows them. Decreased movement rate for 10 seconds, 5 second cooldown. Stacks. 2000 reiki to use it.


Gekijou Shippuu [Fury hurricane] - 1x1 projectile technique. This is the only technique that the bankai possesses before being mastered. 2 second cooldown. Uses 250 reiki. x1.5 damage multiplier, based on reiatsu power.

Charge - adds paralysis status effect to zanpaktou-related projectiles, takes 2000 reiki, removes all reiatsu-power boosts and doubles attack, while keeping the x2 defense boost from bankai. Lasts until turned off.

All techniques from shikai are still usable in bankai.