Triglavians invading Warframe?
Dammit, I've had an idea like this for a while now but no time to work on it! I've always wanted to see the enemy factions use better tactics instead of just getting bigger numbers until they're basically immune to everything and one-shot anything that isn't invisible.
If I might make a suggestion re: dedicated melee units, I think they could use something of a rework as well. At least for the Grineer and Corpus, the Infested have the opposite problem.
Grineer can afford to use melee units because they grow clones in job lots, so sending people in to fight an Infested or a Tenno in melee combat is actually a viable option instead of a waste. In addition, some of the clones are probably too aggressive to hang back and shoot the enemy no matter how much you train them, so giving them a machete or cleaver and sending them forward in a charge is probably the best way to use them.
Now, with that said, as much as Corpus propaganda would like you to believe otherwise the Grineer are not complete idiots, and any melee units they deploy are going to be equipped in such a way that they are as effective as possible. Guns replaced melee weapons for a reason, and charging someone head-on while they shoot you full of holes is just stupid. Any melee units should have a bulletproof shield like the Shield Lancer, a jetpack like the Hellion or Reaver, teleport like a Flameblade, or ambush you like a Manic.
As for the Corpus, since they make heavy use of proxies instead of industrial-scale cloning facilities like the Grineer everyone they train to fight is much less replaceable than Grineer soldiers are (and probably better educated, too). As such, the Corpus generally refuse to engage in melee combat unless they have no other choice, instead preferring to blast their enemies at range from behind a wall of expendable proxies. Prod Crewmen should be given a short range weapon like the Quanta, Glaxion, or Amprex and renamed appropriately to reflect this (that this makes John Prodman even more of a badass is just gravy).
Every Corpus unit is armed with their primary weapon, a Spectra as a sidearm/utility weapon, and (depending on the environment) a Prova/Kreska/both as a backup melee weapon. Units that need to get close for their abilities to work like Nullifiers, Comba, and Scrambus will use short range weapons like the Sonicor, Convectrix, or Plasmor. Since they're such a game-changing unit for fighting the Tenno, Nullifiers in particular ought to be treated like rare and valuable assets, equipped with the best technology the Corpus can buy and armed with their most expensive and powerful short-ranged weapons, such as the Exergis or Arca Plasmor.
Any dedicated melee troops should be elite units only found at higher levels like Comba and Scrambus and equipped with every possible advantage such as hover tech, jetpacks, nullifier bubbles, etc. to maximize the chances of such a risky investment paying off, and their lore should reflect how the rest of the Corpus' military forces think they're nuts. Corpus melee weapons should also reflect this, with the Prova and Kreska being utility weapons while the Lecta, Galvacord, and Falcor can strike from range. Any deviation from this formula needs justification in the lore, which is already the case for several weapons:
The Ohma are used only by Tia Mayn, and quite obviously custom weapons designed for her like Derim Zahn's Ferrox. We just got our hands on the blueprints.
The Serro was originally designed as a tool to scrap obsolete ships but was quickly banned after rebels discovered it could be used as a weapon. That the rebels were in the Sedna region implies it was sold to the Grineer for use by their slaves while the Corpus had a laugh at the Grineer's foolishness for not using automated shipyards, then laughed even harder as violence ensued.
The Obex are constructed using Corpus technology, but their lore states that they were designed by the Tenno. This might also be the origin of the Falcor since it's described as a (emphasis mine) "Corpus-tech glaive" instead of "Corpus-designed glaive" or just "Corpus glaive".
The only outlier here is the Arca Titron, and I can see that being a weapon intended for jumping shock troops like the Trencher or another custom job like the Ohma. Time will tell what DE chooses, but I'd lean slightly towards the latter.
Anyway, that's just my advice/opinion, but feedback would certainly be appreciated! I like what you're doing here, and I might make my own thread if I can figure out how to rip weapon and unit models from the game and import them into SFM or GMOD. Being able to actually show what I want a unit to look like without relying on badly-photoshopped screenshots would certainly be nice...