One name that cropped up several times while reading about other French composer was Vincent d'Indy. He sounded quite important and influential but to be honest I'd not heard of him before. Back to the research.
Firstly, I just have to say he had a fantastic moustache, the ideal 19th century Frenchman. Which is fitting as he sounds like a very patriotic Parisian. His most famous work is the Symphony on French Mountain Air, with some folk tunes as a base, and it really has that mountain countryside feel. I guess a bit of a predecessor to Strauss's Alpine Symphony, but not as dramatic, probably more pastoral really. What's really interesting about it is it uses a piano, so it has some resemblance to a piano concerto, but the piano works with the orchestra in a much more integrated way. This should get played more - I hope people vote for it in the countdown.
The reason he keeps turning up however is his influence as a teacher. Along with a couple of others - and inspired by his admiration for his teacher, Cesar Franck - he founded a second music school in Paris, the Schola Cantorum de Paris. The famous Paris Conservatory had become almost entirely focused on opera, so even Franck essentially didn't fit there. d'Indy and his fellows were much more interested in orchestral work so the school provided that focus. He had many students who were successful composers in the late 19th/early 20th century, including Satie, Milhaud and it seems Cole Porter which is interesting.
He was a huge admirer of Wagner and there's some signs of that in his work - I'm told, but I can hear it in the brass section of the Symphony on French Mountain Air. He also seems to have been quite forward looking in his composition, so a pioneer of the 20th century revamp on Romanticism - there's no doubt his music still falls into that classification though. Take his symphonic variations, Istar for instance. I can't find any more info on the piece but I'm assuming it's inspired by the Assyro-Babylonian goddess of the same name, the one of love and war, but more her loving side.
Well worth exploring further and clearly an influential man even if he has become somewhat obscure now.