It could be any or all of the following:
Shin
Waite
Water
Wine
Will (representing the Sword in the Cup)
Wafer
Work (as in Great Work)
Word (as in "the Word was made flesh" & Christ as the True Word)
Woman
West (direction of Water)
World (spirit into matter)
It could also be an M when viewed from the perspective of the Above descending (just as the dove is descending). From the Divine perspective it could be:
Mem
Mary
Mother
Mystery/Mysticism
Micro/Macrocosm
Matter
Magus/Magi
Do not forget that Waite was an expert on the Grail legends, and he wrote The Hidden Church Of The Holy Grail in 1909. In that book, he has a chapter on "The Parsifal of Wolfram von Eschenbach," the most esoteric and heterodox version of the story, popular in central Europe in the early 1200's.
In that chapter he refers to Wolfram's name for the Grail castle-- "Mont Salvage." So, the Grail is kept hidden away in the castle of "Mont Salvage." And it so happens that the typeface for W/M on the card and the typeface for W/M in the book are identical. Some kind of old English typeface. (The vertical right bar is wider than the vertical left bar on both, when the card's letter is turned upside down into an M. The inside "V" of the "M" has the same unusual thickness. To my sight, the Grail in the Ace of Cups is an upside -down "M" as found in the same typeface in Waite's book on Grail history. It could stand for "Mystery," another favorite capitalized keyword in Waite's book, along with "Mont Salvage," the Grail castle.) ) Maybe coincidental, maybe not. But let's include where the Grail is hidden away, "Mont Salvage", as at least a possibility in the list. That is where the Grail heals.
One other observational speculation. From the earth below, the Grail shows a distinct "W." But from the dove's heavenward view, the Grail shows an "M." Thus, we may be seeing two complementary letters here. From the earth we have the "Word" beyond the world, while from heaven we have the castle to which the bird descends, "Mont Salvage." -- In the Wolfram story, the dove descends from heaven with a wafer, empowering the Grail in the castle with it. So the life-waters outpour to the world below. Maybe the letter is double?