hello,

do you know any sites that proposes new maps to download for this very good game?

Unfortunatly not sir.

The only site that I knew of, don't know if its still there is etherplanes.
It started off fine with duel decks and a few downloadable maps, alas it died a death.

The game, afaik has tools to create campaigns though to my knowledge none were created.

Much like me you are stuck with playing the campaign, sorry.