I haven't seen much 3rd party support for this cooker, drop a line and share some of your favorite accessories and must haves for the Summit Charcoal!!!
You won't need much for the Summit. No need to buy a hinge or a stainless grate. Already has an "easy spin" grate because the kettle tabs that you have on the regular kettle to support the grate to don exist on the summit. The popular temp controllers work with the built in ATC port.
No need for smoker blankets, door upgrades or adding BBQ gaskets either. It is already insulated and sealed.
While I do have most of the GBS accessories and use them from time to time, I could probably live without them and have a grate without the hole in the middle. Not sure anyone makes stainless grate ye for the Summit but i think there may be at least two options for cast iron if you want that.
Most handy accessories for me so far have been the Thermopen and Smoke (or any other similar monitor with similar range - not blue tooth). I find myself not using automatic temperature control as much as I did when i first got the summit. In fact, I find myself not using ATC at all anymore on the Summit because of how efficient and stable it is with the temps. Being able to keep an eye on temps remotely with the Smoke is enough for me.
With the lid bolted to the hinge, you can't really use a Santa maria attachment even if one existed with the lid and hinge there.
With the charcoal grate that you can raise and lower and the charcoal baskets, I would argue that you do not need a Vortex or SNS type accessory either. With that top flipped open, the Summit really gets those temps up there in a hurry.
A rotisserie would be nice and along the lines of the Kamado Joe one but with the ability to do a bear can chicken (stainless GBS adapter) or just doing spatchcock, the need for a roti is somewhat diminished though I would still like to have one for my 22" kettle (Performer).
IMHO, the Summit gives you a lot of functionality and flexibility right out of the box so has less need for accessories.