If you're looking for some modern, interesting, atonal music written exclusively for Euphonium, I personally think Kenneth Friedrich's music is right up your alley.
Check out some sound samples
here
If you like what you hear check out his sales page
here
Personally I recommend Prayer, Air No. 12, One Moment, Air No. 15,16 and 24.
Though if you're looking for something more atonal still, check out Air No. 4, Ma Vie Comme Une Mouche, Can My Heart Love Again, Air No. 6 and Departure. Not my personal favorites (I'm not big on atonal) but I hear lots of good things about them from those who do like them.
His Concertos might just be up your alley as well (though they are rather difficult).
I also recommend anything by Philip Sparke (Concerto, Fantasy, Harlequin, Pantomime, A Song for Ina--just a few examples). Sparke's pieces aren't really atonal but they're extremely catchy and make great openers and closers to your performance.