As a trombone player, I can tell you that the solo literature is full of good examples. In particular, music intended to be played by advanced high school/early University students would be a good place to start. These pieces show the "gist" of how trombone cadenzas tend to be written, but since they are aimed at not-yet-pro musicians they are very idiomatic. A close second would be to look at classical period trumpet concertos. Many of these, transposed down a major ninth (if they appear for trumpet in Bb), will be very doable on trombone.
Best yet, if you can, see if you can spend an hour with a university level trombone instructor. He/she can play several cadenzas for you and point you towards a good source of sheet music.
As an example, this is a cadenza from a very common - ubiquitous, even - solo piece called Morceau Symphonique (Alexandre Guilmant).

I use this as an example because it's flashy and "showy," ending on a high Bb, which is a triumphant-sounding note. As cadenzas go, however, it does several things that make it good for trombonists. It first is a series of runs in Bb major (the trombone's home key), and then an elongated Bb7 sonority. Both of these feature notes that are convenient and idiomatic for trombonists. More difficult cadenzas might be set in different key areas, end on a higher note, be longer, etc. Any of these things would make the cadenza significantly more difficult to play. If you were to compose this exact same idea in A instead, it would be more difficult because the slide positions are less idiomatic in sharp keys (at least for intermediate players).
There's no problem with keeping the cadenza at this level in my opinion, either. A soloist could always choose to up the ante and extend it, or add in some whiz-bang effect, or play a few extra notes in the stratosphere to dazzle and amaze...