Whatever else it might be, There cannot be a subject. There is an adverb and is locative (usually emphasised) or existential (usually unemphasised).
"There is a cat in the garden" / "There are cats in the garden." -- "A cat is in the garden" / "Cats are in the garden."
Notice the subject verb agreement, and the inversion caused by the fronting phrase, "there". This is a vestige of the earlier Complement-V-S inversion
Dearly did I love her but long was the road to her heart."
This itself a vestige of "the verb as second element."
The unemphasised "there" is existential:
A: Unicorns are mythical.
B: No! There are unicorns. = No! Unicorns exist.
"**There * is a cat in the garden" = a cat *exists in the garden." thus giving meaning to the otherwise lexically empty "is".
Compare:
A: “We have no actors who can play the hero.”
B: “That’s not true: there [existential] is John.” = “That’s not true: John exists (as a person to play the hero).” NB inversion.
A: “But John is in France.”
B: “No he’s not. [points] He is there [locative] near the coffee machine.” NB no inversion.