Will it be possible to daisy chain a NXT brick as a slave of a new EV3 brick ?
As per This LEGO education FAQ, it is not possible to use the USB cable the same way it's done between multiple EV3 bricks:
Can I connect my NXT Intelligent Brick to the LEGO MINDSTORMS Education EV3 Intelligent Brick? No. It is not possible to daisy chain with the NXT brick.
However, keep in mind this is valid within the scope of official software; it could be that someone later makes the necessary hacks for it to work in some way.
Also, other communication methods such as bluetooth are available which you can likely use to control the NXT brick from an EV3 one, rpovided you program the NXT brick accordingly. It probably won't be as simple, but definitely doable.