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The last stop on track 1 is games that introduce 2 new pieces, generally on
an 8x10 or 10x10 board. The 'traditional' chess expansions, the ones that
people do/will/may play, have only 2 pieces difference from western chess.
And these 2 pieces are additions to the standard KQRBNP, not replacements
for them, in general. Specifically, the variants of
Carrera-Capablanca-Grand Chess are the most likely form[s] of the 'next
chess'. Fergus Duniho's Eurasian Chess is an outlier of this 'last stop
for next chess' - the pieces that are acceptable are quite constrained.
The Amazon [QN] is too powerful for track 1; the [BN] and [RN] are
acceptable, and the Vao and Pao [Fergus' added pieces in Eurasian] are
also, but just barely. 

Once you get to 3 different pieces in a variant, there is no chance that
it will/could become 'the next chess'. From here on out, all stops are
on some spur of track 2, the 'non-serious' chess variants. [Need I say
that I find these far and away the most interesting? :-) ] None of these
games is trying to 'improve' or re-work western chess. They are flat-out
explorations of what chess[like] pieces and chess[like] rules are capable
of doing. 

And time has marched on, other comments are accumulating, so this ends in
a few sentences. As George has suggested, we can organize the wilder side
a bit. Patterns of design and new ideas are starting to emerge from the
fringes. This is where the adventure is. I'd like to map out a couple
paths to see in which directions they are going. [Btw, George, we both
agree, FRC or any 960 variant is a track 1, and probably the first thing
FIDE will actually try next - I'd listed Switching and Extinction as
track 2s. The impression I got from my 1 game of switching is that there
are no already-learned chess strategies you can apply to Switching, so I
figure it's too outre.]