The Elites section of the Tyranid codex is where I feel we need a lot of improvement. First off there's not a lot of selection. You can fill an entire FOC with every single unit in the codex. What I love about the Eldar codex is the sheer choice of units- you can never take everything in the codex in one game. Of course Tyranids have a lot of selection with our "build a bug" biomorphs, but I'd like to have more bugs to start with.
Lictors: Oh how I love Lictors but are they expensive. Knowing I have a Lictor makes my opponent be a little more careful about parking his vehicles in or near area terrain. The problem is they are glass cannons. Even a 5-man Marine squad can take one out easily (especially if the Sgt. has a powerfist). They'll suffer a few casualties to be sure, but your 80 point Lictor is now gone. Of course Lictors need to be supported, but with rolling for reserves they may not pop up when you need them. What I'd like to see with Lictors are three things: firstly decrease their points, secondly don't make them roll a dangerous terrain check when they pop up, thirdly allow the Tyranid player to choose when they come in. This could be done in a few ways, the first being to simply put them in when you want them. Another could be to write down before the game what turn they are going to appear. Or even use a Spacehulk-type "blips" system to show where the Lictor might be.
Tyranid Warriors: I am actually quite satisfied with these guys as is. In 5th Edition they are one of our strongest choices when kept cheap and armed with Deathspitters. I only give them extended carapace and toxin sacs as far as upgrades go. I don't give them Enhanced Senses because you are simply rolling to scatter. A six man unit can put out a lot of hurt on anything. Even if a lot of shots scatter off, you still inflict a lot of S6 wounds. The one area of improvement for these guys is I'd like to see them get more psychic powers than just Synapse. I can see them using Catalyst to great effect, or even a completely new power.
On a side note, if you arm your warriors all differently, you can pull an ersatz Nobz Bikerz trick. Of course you would have a very ineffective unit, but it is a funny idea.
Elite Carnifexes: I'll go over the 'fex in the Heavy Support edition.