Dear All, Well, I am not sure that I would be good at this kind of game...
However, I have a few remarks/questions.

While Brahms's 1st symphony obviously has a Beethovenian air about it, I
cannot see how Schubert's Great C-major symphony could be connected with
Beethoven.  Of course, it is of a similar level, but what else?

And Beethoven, if I remember correctly, conducted the premiere of his
Symphonies Nos.  7 and 8 almost completely deaf.  Needless to say, there
were gigantic piano sonatas yet to come, major string quartets, Symphony
No.  9...

Isn't Mendelssohn's 2d violin concerto one of his truly great works?

About string quartets...  Please do not forget Stenhammar...  and a few
others.

Now, just a thought.  Some composers have not been quoted because they are
not considered as having passed away so young.  But what does it really
mean? For instance, Bartok left while he had tons of projects and ideas
in his mind.  He was 64, like Brahms, but not in the same state of mind,
probably.  I can quote a few composers who brought us a lot after they
passed their 64th year.  Vaughan-Williams (5 symphonies), Havergal Brian
(! 23 symphonies, I guess), but also Faure, Frank Martin.

All right, I know, it somewhat spoils the game.  Sorry!

So I shall add some composers who really died young.  Lekeu, Bizet, Goetz,
Vivier, Wilhelm Fredrik Bon, Grassi, Bo Linde, Arvid Kleven (a very
remarkable composer),...

Best wishes,

Thanh-Tam Le