Carnifex in place of regeneration:
Gains a 4 + cover save against shooting due to its big size and stuff. "Oh I shot it in the head ! , nope, that is just a big bit a alien ceramic horn"
Genestealers (not ymgarl) - gain the ability to appear anywhere on the board more than 12 away from an enemy unit after coming out of reserve. May move and assault as normal. For all the haters, this effectively just means they get to assault then die, rather than just die. Also still lack grenades.
Trygons - for extra points - (maybe 25) may assault and fleet the turn they arrive via deepstrike. This just gives the "oh wow" factor that many units give from other armies that nids lack in place of - "lol just shoot it".
Lictors - may assault the turn they arrive.
Endless swarm special rule - all hormagaunts and gaunts gain a 5+ cover save when within 6 of another gaunt or hormagaunt unit and any non TMC unit within 6 of a tethered unit also gains the save.
All models within synapse range gain the preferred enemy special rule to reflect that this should be a bonus and not a hindrance.
Then you have a codex which would be tier 1 and might stand up to the big boys. Not over powered because you still have basically no way to get through opponents tanks. Probably still weaker than space wolves as stuff is still absurdly expensive.
Ok so I won't propose these as what should be in the codex as I know how the minds of people work and they won't accept a buff which would make the army threatening to them but that is how weak I view the current tyranids codex it is so lacking that it would need the above buffs to be viable to win against serious opponents of equal skill.