Ok, then let's wait and see ...
Hans
Wait no more!

Great idea for a 20-year project! To order the 555 Scarlatti sonatas in order of difficulty!
In the meantime, you can have a look here where I have graded and commented on over 100 sonatas:
https://pianoforum.net/smf/index.php/topic,2339.msg20064.html#msg20064(favourite sonatas).
And here are 192 (I think) sonatas ordered by difficulty

. They are all superb pieces of music, but I have underlined my favourites (reflecting my personal taste rather than the excellency of the sonata). And of course, degrees of difficulty are always subjective. I would also suggest that you always learn in tandem a fast, virtuoso sonata (usually marked allegro and above) and a lyrical, slow one (usually marked andante and below). Enjoy!
Elementary sonatas:
Grade 3:
K32 - K34 –
K40 - K42 - K73 - K80 - K81 - K83 - K88 – K382 - K471.
Grade 4:
K63 - K74 - K75 - K95 - K163 - K199 –
K322 –
K377 - K512
Intermediate sonatas:
Grade 5:
K25 –
K54 –
K59 - K65 –
K67 -
K70 –
K79 - K85 - K102 –
K112 –
K123 –
K175 -K178 –
K201 –
K203 –
K208 - K211 –
K213 –
K274 - K278 - K285 - K307 - K323 - K325 - K335 - K341 –
K342 - K348 - K349 –
K373 - K376 –
K381 - K389 - K394 –
K402 –
K404 –
K405 - K453 –
K454 –
K461 - K514 - K515 –
K531 - K541 –
K544 –
K547 Grade 6:
K1 - K11 -
K13 -
K19 -
K20 -
K35 - K38 - K51 - K53 - K64 -
K77 - K100 - K105 - K109 -
K119 - K125 -
K132 - K146 - K149 - K158 - K165 - K184 - K193 -
K198 -
K212 - K214 - K239 -
K247 - K255 - K264 - K266 - K268 - K289 - K300 - K303 - K333 - K371 - K378 - K379 - K 380 - K384 - K386 - K387 - K391 -
K421 - K425 -
K427 - K428 - K429 - K431 - K432 - K440 - K446 - K470 - K475 - K481 - K490 -
K491 - K502 -
K511 - K520 - K532 - K537 - K551
Advanced sonatas:
Grade 7:
K2 - K4 - K6 - K8 - K9 - K33 –
K39 –
K46 –
K52 –
K69 - K82 –
K98 - K124 - K133 - K134 –
K135 - K140 - K159 –
K197 - K200 –
K259 - K261 - K284 - K318 - K393 - K415 - K436 –
K443 - K487 - K492 - K496 - K507 - K525 –
Grade 8:
K3 –
K24 –
K27 - K29 - K30 - K31 - K41 - K58 - K87 - K93 - K96 - K113 - K120 - K128 –
K141 - K142 - K162 - K169 - K185 - K417 –
K426 - K430 - K444 –
K466 - K474 –
K478 - K513 - K517 - K535
Best wishes,
Bernhard.