I actually share similar interest with you in those big saloons so you are definitely not the only one standing alone 😁
Yea you are right, they pretty covered all the S classes (since the official designation of S class started with W116) all but the W221... Personally i thought W221 looks better than W220 and anedoctal evidence shows that W221 fares better than W220 in term of long term reliability? Then again it doesnt really matter for most of us as any faults with out of warranty S class can easily leaves us in tears, and a broken man (or woman)
I think they should definitely consider W221 at some point. They are even making the old W111 now, and they did a W186 300C which i believe is the S class predecessor long ago, likely re-used the same mould from Ricko?
I secretly hope they will consider a W203 in the near future although i think that is the least popular C class of among others.