The single biggest thing I want to see for a new Tyranid codex is to massively cut down on the redundancies of the current codex. I would love to see everything have a place without doubling up. Examples:
Tyrannofex -> Carnifex: The Tyrannofex is a Sniper Fex from last edition (Sniper Fex was T7, W5, 2+ save with a Venom cannon and barbed strangler when the venom cannon had the 100% exact same stats as a rupture cannon). I want to see the Tyrannofex fill a different roll as right now they are both kind of ghetto gun beasts that are obscenely expensive.
Ymgarls -> Lictors: So...a unit that picks a piece of terrain, comes in reserve by popping out of terrain and can assault? Sounds a heck of a lot like Lictors from the 3rd and 4th ed codex. I mean, Ymgarls are just mini lictors. I would love to see Ymgarls removed and Lictors beefed up a small bit and given Ymgarls deployment.
I just want every unit to feel like they have a place instead of being made totally redundant by something that is just OBVIOUSLY better. The range section also totally fits in this as you could lose half the weapons and nobody would notice. FOCUS PEOPLE!! FOCUS! lol. Regardless I hope for great things with the new book!