Your go, Ms Void!!
Headcanon about Arthur (or Freya??)
Agh. I always take so long to come up with headcanons lmao
I've already done a couple on Arthur, but hmmm, let's see... Freya...
In my world, if Freya survived, she would be able to change in and out of her Bastet form whenever she liked (except for a few limitations like maybe she has to shift at least once a week or something because the curse is just so intertwined with her body).
My biggest headcanon that also fits in with canon is that the Bastet curse was taking up all her magical abilities which is why we don't see her doing any magic in Merlin. She used to have the magical abilities of an average person (say a step below Gaius's power), but when she was cursed, the curse caused her magic to turn against her and regularly turn her into a bloodthirsty-Bastet each night.
Which is why Merlin was the only one all this time who was able to calm her down in her Bastet form. 1) Because she loved Merlin and his soft, gentle and reassuring ways deeply and also 2) because Merlin is Emrys, he is magic and so he was able to reach out to Freya's magic through their connection with one another and calm it down.
And before the Lake of Avalon, while Merlin was sobbing and whispering for her to not let go, his magic listened and made Freya the Lady of the Lake. The Triple Goddess saw what Merlin was (subconsciously) doing and since he's Emrys, she trusted his reasoning and helped with the process to make Freya the Guardian of Avalon.
That's my two cents!