Will there be new Codexes for 9th edition?
As Warhammer Community just revealed the release of codexes through January of 2021. With all the new Marine minis coming out it only makes sense they get the next two books. After that though, some other factions are getting some love!
Can tyranids speak?
They speak to humans using telepathy, and they can now ally with Space Wolves as battle brothers, because reasons.
Can chaos be defeated?
Short version: the more mortals push the universe, the more it pushes back. This means that without mortals, the chaos gods die, or exist in such a weak form they have no effect on the materium. The only true way to defeat chaos is to eradicate all sentient(or in general) life in the universe. But you can.
Can the necrons defeat chaos?
If you start by assuming that the necrons have an ultimate weapon that can destroy the eye of terror, or the warp itself, then sure they can probably beat the forces of chaos.