I’ve wrapped the passenger airbag in leather to hide cracks and dyed the roof black but it still feels a bit dull

 

It's a matter of taste isn't it. I like the stock interior because it's simple and elegant. If you wanted minor improvements you can try to score R3 seats and accent pieces. I don't know that they're more comfortable than the original leather but definitely look better. Depends on your size.

If you want more oomph, extra lighting is always fun to play with.

 

It's a question of taste but the S2 steering wheel might be a nice update. I changed the leather shift boot with a 4 panel one with a black embroidered RX-8 logo with a new shift ****. Subtle changes but gives a cleaner look. I find the stock seats just fine. A few month ago I tried an R3 and did not find them as comfortable. Must be getting too old...

 

The Autoexe steering wheel is nice if you want something a bit thicker to hold with thumb rests. An Android Auto head unit is not too difficult of a job. Oh and get some matte black bumper paint for the piano black trim.

 

I gotta carbon fiber wrap mine. Its definitely seen better days. My only concern is the wrap potentially blocking the airbag.