Actually, I can find no mention of the original
work being a theme-and-variations work (perhaps
someone with a copy of the original, Caprice No. 24, can
testify to that).
At any rate, while Rachmaninov's
Second Concerto is probably (now) his most often-heard
work, I think many musicians (and Rachmaninov himself)
would rate his (five!) symphonies to be his most
important contributions to music.
On the
question-writing front, as somebody who writes a lot of questions
about musical works, I find it excruciatingly difficult
to say that a work is "central" to a composer's
output. There is simply too much critical disagreement to
make such a claim as "X is this composer's major
work."
At the same time, however, it is much easier to
write questions about how particular works broke with
tradition, have unusual structures or provenances, or
represent influences from other composers. As a result, a
lot of the questions I write that ask about
particular works require (at least early on), a significant
amount of detailed knowledge of the specific structure
of the work being asked, and of the details
surrounding its creation.
--AEI