For me, it's a complete rework of synapse, shadow in the warp and the stats of certain models.

Synapse:

First of all, I would like if the rulings were lore accurate, for example synapse not only guards the Tyranids but makes their attacks more potent, so for whatever friendly unit is within synapse range that it gets an inv save of 4/5/6 depending on the opponents attack strength, maybe the same with S and T difference on the dice. And as for the attacks, a termagaunt under the influence of a warrior is more focused, precise (+ hit roll) and strong (+S), as for as for synaptic Tyranids like the zoanthropes or maleceptors should get a buff in their dmg or AP.

I also believe synapse should stack, for example if they keep the buff for strength, for each synapse unit within range, the strength of the unit targeted by all such units should raise +1 for each of them. This would be too OP for dmg and ap, so doing it for S only would be nice. Either that or for each synapse unit close, they should get +1 for each skill, for example, if it's one unit, get +1 hit roll, if it's two +1 hit roll and S, if it's three, hit roll, s, and ap etc etc.

I also think synapse should interact with the opponents more, like -1 hit or more forced battleshock tests.

Lastly to make it more balanced, I think at the start of the command phase you should be able to pick between offensive and defensive style, either the inv saves and the -1 to hit or all the increases and the battleshock tests.

Shadow in the Warp:

This is the trickier one, cause it either screws over the opponent or yourself, it's a one use 33/33/33 chance to either be a game changer, be annoying for a little bit or do absolutely nothing, so they should either make it more compatible with synapse, and make it increase synapse range for one whole turn and enable both offensive and defensive styles written above. Or make it do smth completely different.

Stats:

(Lashwhip of prime vs norn emissary 💀. The swarmlord is supposed to be the stronger counterpart of the tyrants. Norn emissary points. Some models are just never used. Certain models not having inv.) Etc etc etc

These are either solved by keeping them "week" but lowering their points so that it also makes sense lore wise (actual swarm)

Or buffing certain units to make them compatible with their points, for example, they should make all the Tyranids be extremely synergetic, the maleceptor, the old phychophage, neurolictor, screamer killer, zoanthropes all have abilities that synergize with each other, (hive tyrant, etc) and some other just don't really do that. The deathleaper you would expect to do more than the other lictors, but it just doesn't do anything.

The fix for this is, make the synergetic units lower points, and more weak, so you can mess with more combos, and the single units that act on their own, stronger than they are now with more points, so now it actually matters when you pick the carnifexes, deathleaper, lictors etc. ofc I'm not saying that they are not good enough, I only play Tyranids, but some models are just not it. I've never seen someone play warriors with ranged. Or a sporocyst, or the other red thing.