Instead of NAT traversal IPv6 is going to require firewall traversal.
Your device also isn't hidden too much if a NAT router lets it have the ability to make outgoing connections - it's just less convenient to access.
You might see more attacks targeting specific internal IPs if NAT never existed more but it doesn't necessarily follow that NAT is preventing or reducing botnets. Dos/DDoS attacks can be focused a your router with or without NAT.
Problems start with NATs that you can't control.