While each person's vision of their roadster is unique, I think most of them can be classified under styling genres.
So far this is what I've come up with with a brief overall description. Feel free to add to it.
All Original - Owners perform no modifications to their car. This encapsulates most Miatas as most folks just drive their cars. The enthiusiasts in this group usually have garage queens and take pride in how original their cars are and often feel they will hold their value better as collector items. The cars also often have low miles on them.
Classic Roadster - Owners modify their cars with bits of chrome, wood, leather and/or bodykits inspired by roadsters of the 50s and 60s. Also referred to as Vintage.
Autocrossers - Owners only perform mods based on SCCA rules and typically only if they offer performance benefits.
Spec Miata - Owners with dedicated track Miatas modified for Spec Miata racing.
Tuners - Owners who only modify their vehicles performance and see cosmetic mods as unnecessary or added weight. Often these cars are boosted and include full coil-over suspensions, big brakes, programable ECUs, etc... These cars are street driven and also track driven. Since they don't autocross they are not restricted by rules and since they are street driven they are more comfortable than Spec Miatas.
Roadster on a Diet - Owners of these cars are obcessed with shedding as much weight as possible. Gutted interiors, no A/C or radio. Often no top at all. Body panels may be swapped for carbon fiber duplicates. They go to more extremes than the Spec Miatas in saving weight.
JDM Lovers - Owners modify their cars with rare and hard to get Japanese home market products.
Full Show Cars - Over the top cars with plenty of bling. They have body kits, large rims, custom paint, killer sound systems, and lots of shiney bits. At the extreme level these are not street driven but trailered to shows.
Club Roadster So-Cal Style - Seems to be a very specific style. Standard requirements include a hardtop, slammed suspension, wide zero-offset rims with narrow tyres stretched over the rims and fender flares.
Naturally Miatas can also fall into non-specific vehicle genres like low-riders, rat rod, etc but I was trying to break down the Miata specific genres. I might be missing out on some so feel free to add to this.
What genre(s) do you fall into or have in mind as your style direction?
I personally consider my car to fall into Classic and JDM but aspire for some of the added performance of the Tuners and maybe some of the engine bling of the Show Cars someday.