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Indeed, unifying Chess960 and Chess this way is a nice concept. But it only
works because these games nearly are the same variant. You acknowledge
yourself that you already run into problems with Janus vs CRC, which,seen
from the viewpoint of a Xiangqi or Shogi player are practically the same
game. But the promotion rules are slightly different, as are the castling
rules.

The promotion rules could be attributed as a property of the Pawn, and in
this view a CRC Pawn and a Janus Pawn are different pieces. This becomes
more obtuse in Chaturanga, where the promotion is determined by the board
square you promote on, and thus can no longer be considered a property of
the Pawn.

And how about Losers Chess vs normal Chess? How could you recognize that a
FEN represents a poition from Loser Chess rather than normal Chess. How
would you see it from the PGN if the variant tag was merely a comment? The
game might end with a resign, so the absence of checkmate might not be
apparent.

Your unified approach simply does not work when the variants differ more
than a trifle, or becomes exceedingly cumbersome. WinBoard aims at
supporting a wide variety of variants. A really universal FEN standard
should be able to handle variants the designer of the standard did not
even know. This is why X-FEN is unacceptable for use in WinBoard, both in
communication with the user and with the engine. It does a lousy job
representing Xiangqi and Shogi positons....