So I was watching August's stream and he explained rather well one of the things that is going to hold her back as an ADC is a very long attack point which "basically roots her"..
Attack Point isn't a stat that's talked about a lot, you won't find it in the wiki, but basically when somebody starts to attack they have a few frames before the attack occurs (aka frontswing), Xayah for instance has a 10 frame attack frame, which is considerably quick, and these values are decreased by attackspeed (meaning xayah gets down to basically instant attack startup).
One part of League where Attack Point is mentioned is Thresh, who's passive reads attack "wind-up"(attack point) is increased by only 25% of every 1% attackspeed (0.25%), so no matter how much attackspeed Tresh builds, he's always slowed down.
So what does this mean for Senna? First her attackspeed efficiency is awful, Lethal Tempo+Rageblade+5 witts End only gets you to 1.63. And because the attack point is so slow, combined with not much attackspeed to speed it up, if you try and attack move quickly you'll actually end up cancelling autos because she doesn't finish her animation. You actually have to root yourself to auto even in the best case scenario.
So yeah, I don't think this is going to be some busted ADC at all. Combining all this with reduced critical damage I'm not saying she won't be able to do the role, but I don't think Riot is going to have to restrict items on her like the OP suggested..