I've been heavy into motorcycles and Jeeps in life and those are hi-gear talk mod(ification) oriented forums for ideas, beyond should trombonists wave to one another in passing, and other "popular" gear talk subjects, should there be interest to run the other direction here.

1. What farkles have you installed on your trombone? These are the little "bling" mods that have some minor purpose, but otherwise allow one to fill pages of "upgrade" brag.
New spit valve washers? Thumb rests? Unfolding coat rack hooks? Beer cup and cell phone holders? Matching music stand mud mats? portable pumps to fix your trombone when it's flat, auxiliary lighting, camping with your trombone, canvas or topless trombone playing?, aftermarket louder horns, slide guards, horn locks and alarms, code readers to tell what's wrong with your trombone and playing, the forever alluring off topic of Marvel Mystery oil in trombones, and of course air-conditioning for trombones?
Pics typically would be of the trombone mod in the garage, or out front in the driveway.
2. Where are you playing your trombone today (with pics). This is very popular with Jeeps, down to red Jeeps vs blue Jeeps have their own such topics.
Perhaps lacquered vs non-lacquered: where is one playing today, such as along scenic roads, twisties, tailgate parties?
3. Warranty topic for trombone owners. This one gets a lot of traffic. You buy a used trombone on extended warranty, and take to it service. They put one of those service hats on it and while watching Regis inside on TV, the service manager comes in to explain the service will be twice the cost as something is outside the warranty, only to receive the trombone and they've forgotten to take off the protective paper from around the bell, that has a service agent's shoe print on it, and it's still hard to start when you get it home.
4. Speaking of Proper wave etiquette for trombones?, this is often a topic, to ask what is the one that means the conductor is around the corner to cite you for playing too fast? What trombonists do you not wave to and which ones might get mad if you pass them?
5. Cone practice: the essence of course as a topic: How to lean the trombone, 180's, picking up the trombone after it's fallen, and practicing with the trombone in a parking lot with cones, I guess for marching bands. Side note, having returned from Japan, do check out the amazing playing in rhythm/tune while dancing Kyoto Tachibana High School Band on YT.
6. Trombone safety courses. Related to above, these are state sponsored half-day classes with police departments that usually come with coffee and doughnuts. You bring your trombone with gloves and a long-sleeved shirt or jacket and after inspection, which they sometimes do to say your embrasure is not compliant, you enjoy a day of trombone safety practice - such as how to play the trombone when going over uneven stages, defensive trombone playing, dirt trombones and staying on the designated melody when playing outside, is it legal in your state to "rowsplit" and play the trombone between rows (only in CA orchestras - and some two-row orchestras in AZ when playing from a rest measure), and how to "full lock" the trombone and play it in a continuous circle (I loved that one.)
7. Trombone Iron Lip and badges for charity plays: Everyone likes those little badges to pin to one's suits in playing the trombone, and most experienced journey trombonists wear ribbons and badges galore to question what branch they were in? Long players, over some numbers of hours or days, might receive the Iron Lip badge, while groups playing for charity might receive those fun yearly pins and most importantly these also come with coffee and doughnuts - and often with police escort so you can play with no one interfering at intermissions. Sometimes these "charity" events get a little fussy about trombone brand; some Conn events don't like King players attending, that sort of thing, but I try to find the events that involve cigars, as there's something about trombones, cigars, coffee and doughnuts as a perfect alignment of the stars. Happy trombone playing.