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True, the immense power of the Lion could be a concern. In Chu it is worth
over 1.5 Queen, which would put it around 15 pawn units. (Even without the
double-capture power, a WFDAN leaper would be worth about 11 pawns in a
FIDE context.) Perhaps it is indeed a bad idea to use the Lion as Queen
replacement, because that leaves only a Rook as second-strongest piece, and
thus virtually no possibility to trade the Lion for other pieces.

With Queens, there is at least the possibility to trade L for Q+R. One
possibility would be to replace a Knight with a Lion, rather than a Queen
(e.g. on b1/b8). The downside is that this would make an extremely large
'power density' on a relatively small board. (Not worse than Seirawan
Chess, however.)

I don't want to introduce any other unorthodox pieces, and certainly not
upgrade them, as this would create even more power density. All FIDE pieces
(except K and P) can already attack a Lion from a safe distance (i.e.
without exposing themselves to hit-and-run captures).

Perhaps a better design would be to switch to a 9x8 board, with a setup
similar to Angel Chess, replacing the Angel (worth about 13 pawns) by a
Lion. Non-standard boards tend to strongly decrease the willingness of
people to play a variant, however. That would also be an argument against
using 10x8, replacing C and A from Capablanca Chess by L and an extra N.
(To keep the power-density low: L+N ~ C+A, value-wise.)