So my overall post got a bit bigger than i expected so i did a little bit of formatting to make it easier to read. But in order to combine two races into a new race it's vital to look at what makes them similar and what sets them apart.
I'm not a big fan of your big avenger type of unit personally since it really will screw up your balance and it feels counter-intuitive to be forced to lose units to get access to a big guy. especially if the entire race isn't based on some demonic warlock type of narrative. But all the rest gave great ideas to play around with and build upon.
Race Identity: The brothers grimm meet norse mythology
Gilnean is basically The brother Grim fairytales in 19th century British Islands. While Nightelves are taken norse mythology.
Race Identity: Shadowmeld+Night's curse still works like a Roguelike.
Campaign night-elves played a lot like a rogue-like with the shadow-meld being a huge part of their race identity. Where as the worgen have their night-curse. So perhaps give the worgen melee units a pounce skill and the night-elves having their type of invisibility in the night. You can have them show up in semi-large numbers out of nowhere. (which really sells your chosen identity as a ragtag band that tries to survive after Armageddon has basically visited their homeland.) This as a racial Identity works very well together. And allows you to do some nice hit and run type of strategies. And the slipstream blink that i just read about earlier is insanely fun but might give a lot of issues.
Race Identity: Ragtag Band of internal strife between Cats and Dogs
But being thrown together in order to survive with a different subculture tends to create it's own tension and strife inside the Group. So the new hierarchy can be a lot of fun. So you have ample of narrative to play around with. So a strong leader can unite them. But removing that leader (if only temporarily) you get to play around with a thing a called cats and dogs. And we all know how well that plays together. So in terms of narrative you have a lot going for you.
You could explore certain plants that are effectively like catnip or water and cats fear of getting wet, and a Elek/Centaur/dragon graveyard full with bones which force your player to use his catlike nightelves on the graveyard in order not to desecrate the graves by eating the bones. And the Worgen to not get your cats high on catnip.
Race Identity: Semi-Naval Race
You could even consider to make a more naval race that does most of it's training on the sea. And you could perhaps play around with a Ship as a main building with more active skills instead of it taking up a heroslot. Wood and gold has to be harvested on the shore but fish can be granted from fishing in the sea as well. This kinda would work with your narrative as well. Since Your night-elves used to have their ancient being able to move around. But your Gilneans couldn't but Gilneans could be great shipwrights and builders. But having lost access to most ancients You could op for a nice middle ground with a ship orientated race.(which is also considerably original not sure how blizzard's kul tiras would do it however) Even the regular night elves couldn't move their hunter halls or moonwells so it's okay if some building still have to be placed on the land. And thus forces the player to still become somewhat landlocked. You could even chose to make a race skill scorching wounds that start immolating your own units if they are to far away from your ships.
Race identity: Racing
Racing could also become a big a part of your race identity. Both Worgen, Panthers and Ships (perhaps even a flying unit) all play around with being able to do obstacle runs if you do this in shallow waters you could even have all three compete with eachother. and play around with their custom skills, turns rates/movementspeed/Acceleration/max movementspeeds/passable impassable terrain. (ships can sail on land masses(even small), but ground units can't sail across deep ocean water. while air can go across all(just force "checkpoints basically how actually sailing challenges work as well). It could work well as a festival but also as a way to do some dickmeasuring contests for leadership narratively.
Race Identity: Burning of teldrassil
The desire to play around with more firebased magic and it's results can be done through skillsets, but perhaps when exceeding upkeep either buildings or units might start to burn. Druids of the flame and Fire cat form and Fire hawk form. Considering Fandral staghelm tried to stage a coup due to malfurion's passive stance against the Horde which effectively resulted in the burning of Telldrassil. These fanatics have a lot more worth to their words. Isolationism from nightelves and the past of the gilnean walling themselves off and being left to their demise. Kinda allows you to play with going full circle in the narrative as well.
Overall character and skill Synergy.
Immolation works well with slows and disables So getting a "poison slow arrow" on your archers would work well.
Eating trees, setting things on fire (for both distraction while raiding as clearing blocked paths)
Ensnare As well against ranged units as pulling lever and harvesting shallow waters for fish. But ensnare and flamestrike abilities work as well. Ensnaring opponents then getting out of sight works great in hit and run tactics as well.
another thing which might be fun is looting shipwrecks for gold
Also it would ridiculously funny to have a new druidform of the seal/walrus which can swim but for a limited amount of time. (also another unit which could partake in racing events) But it could work with a more naval orientated race.
Sprouting trees as a skill would work both as a disable but also as a resource generation
