Rumor has it that the Tyranid codex is being "worked on" right now, whatever that means. Here are some of the things that I would like to see in the new one. I'll start with the HQ section.
Hive Tyrant: I pretty much like him as is. His biomorphs are what need improving. The most obvious one is the Bonesword. As is, it's fairly useless and Scything Talons are a better option if you are going for CC. I would like to see the Bonesword go back to its original fluff from Second edition and negate invulnerable saves. That would make the Hive Tyrant even more fearsome.
The other Hive Tyrant option that desperately needs improving is Warp Field. A huge chunk of points for a 6+ inv? Pretty ridiculous. It needs to be at least a 5+ for its point cost. I'll cover this more when I talk about Biomorphs and Psychic powers.
Lastly, I'd like the option for a Hive Tyrant to take a Bio-Cannon. Tyranids need some kind of effective anti-tank weaponry since the VC got nerfed. Make it a S10 AP3 Assault X weapon. To keep it limited, only allow Hive Tyrants to take it.
Broodlord: I love my broodlords but they need two things. They need to be able to fleet, and they need better armor. Perhaps they can have the 6+ inv. I wouldn't mind seeing them lose their infiltrate and keeping Scout, but gaining Fleet. Or maybe they need to deploy like a Lictor does (with some clarification on if they need to roll for Dangerous Terrain). Infiltrate just doesn't cut it for me when they are too slow.
Tyranid Warriors: I'll cover these guys in the Elites section.
Finally I'd like to see another HQ option opened up. What it could be is debateable. If GW keeps up with their current trend of HQ determining army special rules, I can possibly see Genestealers made into Elites choices and taking a Broodlord making them Troops. I don't know if I like that idea, but I can see it happening. Some people have suggested the Malanthrope for HQ, and that is a viable possibility, but he is really just a souped up Hive Tyrant with his current stats.
Speaking of the Malanthrope, I love the model but did they ever get his stats wrong. They made him into a CC machine (and a fast one) but I see it from the description as more of a slow, drifting psychic abomination. Perhaps something like that could be a new HQ- a more powerful Zoanthrope.