So I am a new Nid player, Mostly play with my Necrons and Here is my first attempt at a Nid list.
So here is my list
Deathleaper 130pts If you like him, go for it, but I'm completely unsure how useful he is
3 Warriors, 1 venom Cannon 10pts , 1 death spitter 5pts, 1 pair of bone swords 15pts = 120pts I would consider not having such a mixed up squad. Bone swords are nice but only good if the unit is in CC, which means they should be running to get there. So you should probably drop the boneswords in favor of a 2nd death spitter. I also find the barbed strangler better on warriors than the venom cannon.
20 Termagants, Devourers, Toxin sacs=200 I would drop the toxin sacs on both of these and use them mainly for shooting. Toxin sacs only work in CC and this unit shouldn't be charging too many things that survive a few rounds of 60 shots
20 Termagants, Devourers, Toxin sacs=200
20 Hormagaunts, adrenal glands= 140 As stated by others, you should probably have toxin here. Adrenal is good when paired with toxin on hormagaunts, but not so great alone. Plus they already have fleet so you're only gaining the STR bonus, which compared to toxin sacs will only help against vehicles
20 Hormagaunts, adrenal glands= 140
4 Ripper swarms, Spinefists, deep strike= 70 I would drop completely. They are not going to help much with those spinefists and they should definitely not be deepstriking in this list. They, besides maybe the deathleaper, are the only units who can be apart from the rest of the army and are not really a threat
10 Gargoyle, Toxin sacs= 80 I would bump this up to maybe 15 to 20 gargoyles. 10 will never make it, 20 might be too much. It is what I run and as far as presenting multiple threats, it does the trick. Again, not sure if toxin is worth it here.
1 Carnifex, 1 Heavy Venom Cannon 20pts, Regeneration 30pts, Bone mace 15pts= 185 I prefer dual devourers here, but Venom Cannon is solid. Not sure if the bone mace is a good investment as it will be slow to hit combat since it will be shooting that cannon. I would go kitted out for CC all the way or not at all - right now its like the warriors and trying to do too much. Regen is also a waste here. With only 4 wounds, it will most likely be killed before it can regen much. I personally only consider regen on valuable units, 6 wound units, or a brood of fexes where you can take advantage of wound allocation shenanigans
So my thoughts are in Blue.
All that being said, to add to your list I would second the use of venomthropes and zoanthropes (for their psychic potential as well as you need synapse). If I was playing you at 1250, I would focus my STR 8 weapons on those warriors and watch your army fall apart before they even make it to CC. Forcing moral along the way, which the homragaunts and termagants would most likely fail.
All the other suggestions added seem pretty good as well. Since you're already running so many termagants, you could easily bump a brood up to 30 and take a troop tervigon. Which, provides a tough scoring unit as well as the much needed synapse.
I would consider a 2nd squad of 15 gargoyles for more threats that are quick. With a flyrant for your warlord. It is tough, maneuverable, and deadly. Again, it adds synapse.
At 1750 I have been running 5 sources of synapse and have lost at least 3 in all my games. So at 1850, I would suggest trying to find a way to have at least 5 sources. Warriors are your first source, a tervigon makes an excellent 2nd source for this list.