mellos and baris. I agree with more mello solo's but mellos are usually always gonna be grouped in with trumpets unfortunately.
Bari soli's still happen as well just not quite as often. Trombone has just added a plethora of musical repertoire that has been untapped for quite a long time.
Mello and bari soli's have not been replaced, but they've been put on more of the back burner for now. It will probably stay that way until the designers are bored with trombones.
I think about it in this way, since 2013, there has been infinitely more mello and bari features than trombone. DCI started in 1972 and drum corps itself has been going on way before then.
Its gonna take a while for the hype of a new instrument goes down.