Read an interesting discussion pro/contra double-double vs quad floats on the glibc mailing list. To quote: "No one will use
software IEEE binary128 and we don't have hardware binary128." ...so there are clearly points in favor of using double-double as long-float today, but there's also a 2012 paper on a lower-cost FMA for binary128, so maybe we'll actually see hardware for it surfacing before too long? One can hope...
On similar topic, a 16 bit short float would also neat as well ...but again, neither here or now. :)