My oldest son plays Tyranids. Mostly based on leftover genestealers from Space Hulk (the 1990's one). His army looks like this:
HQ
Hive Tyrant
Troops
3 Tyranid warriors
up to 40 genestealers with at least 1 broodlord available (divided up into whatever groups he needs)
3 ripper swarms
19 termagants
For Christmas, I got him another Tyranid Warrior brood (3 warriors), a Carnifex, a Tyranid Prime, and another Broodlord (cause we have a lot of genestealers...) and obviously he needs more termagants. But what else would be really useful? EDIT: I Forgot to include the Zoanthrope I got for him. So add that.
We do not play competitively (obviously), but he and I play against two of his younger brothers (Tau and Orks, and I play Space Marines), so he doesn't have to be able to win tournaments. He actually does okay with what he has, but I'm focusing on making my army better, so I want him to have the tools to be stay on my level.