Although tyranids are my one true love, I have little hope for them in 6th edition. Even with the new book about to come out in a few months, I don't think they will work. Not the way they are.
Tyranids generally use brute strength of statline to survive (toughness and a lot of wounds). But that isn't cutting it any more. They are also supposed to be close range combatants. Pistol and melee range. That doesn't work much anymore either. The prevalence of high rate of fire and high powered shots means few things can cross the board easily.
Tyranid units need to start being faster and more mobile, but synapse requirements have generally done the opposite of that. It forces the tyranids to move as one force, and units that break off to deal with a threat are generally sacrificed.
A squad of warp spiders can easily put five unsaved wounds on a standard tyranid MC. A properly buffed tau unit can put 8 unsaved wounds. And most of these units can do it with good range/mobility that allows them to minimize retaliation. And it isn't uncommon for many of these to now ignore cover, and tyranids will rarely have an invulnerable save.
With the new rule overwatch, and no longer assaulting from reserves, it is getting harder to make into combat with enough to kill units. This is more so true with many of tyranids "glass cannon" style units.
New rumors are making it sound like Tyranids will still be a very psyker reliant army. And more rumors about MCs, which is not the problem for Nids, it is functional infantry instead.
What I would like to see would be an increase in unit mobility/deployment options, unique perks/protections (immune to rending etc), and tricks to making it into combat (stunning opponents, preventing overwatch). But I won't hold my breath that gw is creative enough to do things like this.
What I'm afraid will happen will be a complete style overhaul like deamons. Even if they do better, they won't be the army I fell in love with. So here's hoping something good happens, but I'll keep working on my tau till then.