What on earth shall we build on Senna?

1. Her mana cost of her Q is extremely high. 110 at max level for a basic spell is expensive.
2. Her Q cooldown is high (15 secs) as well but reduced by auto-attacking by 1 sec. But her autos are slow.
3. Because of problem 1 and 2 She literally has nothing much to do after shooting off spells. And I'm quite sure being "Lethality support MF but with more cooldowns" is not the playstyle intended.

So, the logical idea would to rush Reaver which solves the mana issue and CD problem of her Q. And make her autos faster so she can have a higher uptime on her spells. But then comes the problem she gains 20% AD which seems counterproductive if we build attackspeed.

Should be core but what else?