Hrrm, I'll have to think about that question. I like some of the feedback type of add-ons though. I have a friend that uses a real g-suit leggings that inflates then g's are pulled. Also he's tied in seat feedback by hacking a seat massager. Neither of those things are actual products though. Those are more DIY. I think my next big thing is going to be making a sim tub though. I'm batting around ideas on that. Probably will end up doing one of the A-10C tubs because of my Warthog controller. I already have a ACES II replica kit to use with it. There are a bunch of tub designs available over at the ED forums, but again it's more highly DIY stuff.
I don't recall, do you have the Cougar MFDs? Have you put mini displays behind them to enable them as screen bezels? I have a set of them. I've also pondered that as part of the sim tub build.
When I think about it, it is a good point of interest for me though. There should be more peripherals that give feedback to the player.