I'm probably best described as a "squishy polytheist" - I treat all gods as separate beings, especially when I'm dealing with them, but in my head I tend to believe there are some areas of of overlap where the gods are... not the same being, but perhaps not as separate as hard polytheism would insist. Ultimately, I'm just theorizing, but there are some deities that I can't help but associate with Māra.
The easiest of these, and therefore the one I'm going to start with, is Marzanna. Marzanna is a woman of many names, including Morena, Marmora, and, yes, Mara. Marzanna is a Slavic death goddess associated with winter. Moreover, she's associated with the agricultural cycle and specifically with the death of crops in the winter. Each spring, her effigy is destroyed and drowned as a way to ensure spring fertility comes on schedule.
However, this comparison would be pretty one-sided if I stopped there. Māra, it's important to note, does have some death-related hobbies. She looks after the body after we die, in addition to being the agricultural goddess behind the turning of the year.
Her functions extend, after corporal death, also to existence beneath the earth by maintaining the 'Veli' community under the name Mira Māra, the mother of Veli. In the Dainas Velis (sing. of Veli) represents the astral body (doublet) of humans that after death continues an existence 'as long as the sun remains in the sky' (Daina).
- The Origin of the Baltic and Vedic Languages: Baltic Mythology, pg 58
It's a bit like looking at two sides of the same coin - Māra emphasizes the agricultural, growing aspects, while Marzanna emphasizes winter and death, but both demonstrate the same cycle at their hearts.