I get your point, I was thinking of something more along the lines of an exhalted ability but without having to waste energy to use it, just transform by dodging the same way limbo jumps into the rift, his jump to the rift used to be an ability that consumed energy and everyone hated it, they moved it to a custom dodge that regenerates your energy and now all limbo players love it, plus, when it comes to lore I think Valkyr was supposed to be more like an animal if you look at her Gersemi skin, she has a tail.
In relation to the rig and model just use the rig and model of a kavat and scale the model in the engine to a size appropriate for a player, clearly not as tall as a player but not as short as a kavat, something in between that can fit through doors, in relation to animations just use the same idle animations that kavats have and if necesary they can be tweaked a bit but in general the base model just needs to be the one of a normal kavat, they have idle animations, they can jump, they can run, they can attack and I think they can even bullet jump, they just need to link those animations to the existing input commands in the script and increase the movementSpeed float value and they should be well on their way, that's way easier than having to make custom animations for normal warframes, and the model itself can be a tweaked version of a kavat, nobody will mind the similarities and some may even find them charming.
In relation to the weapons just have a bipedal mode and a quadriped mode and have no weapons equipped when you're in quadriped mode, just use a character-specific weapon called beast claws and fangs, something like Garuda's talons that are only available if you cary no melee weapon or Sevagoth's claws that are only available when you're using his exhalted form, then just use the normal kavat attack animations if none can be created, I don't mind the smaller details as long as I get to run around as a quadriped and I'm pretty sure many other people wouldn't mind either as long as we get the first step towards quadripeds or arachnids (no snakes yet, maybe someday, we can all dream).
And finally to cover the Necramech we can all agree that it's the one way that has the biggest potential to actually become a thing but Necramechs can only be used in very specific places and that's not really fun but if it's the only way then I'm cool with it and hopefully we can deploy Necramechs in normal missions eventually, the important thing is to get as many people here as possible so DE eventually finds this forum and hopefully someday opens this door, clearly I'm not the only one that wants this. :D