Well the K series has been around since 2002; it would be good to see another model - but do SEBO really need another cylinder/can vacuum cleaner on the market?
The way the K has been made, though it survives on a 3 litre dust bag, it sits between rivals on dust capacity alone. Bosch and Miele both offer 3.5/4 litre dust bags with their FJM and then the 4.5/5 litre GN series bags, and Bosch also offer a larger size capacity dust bag for their bagged cylinder vacs.
Something in the middle range between the K and the D series could make sales, but then I think SEBO could offer a "Grand Felix" size that could in theory, take over from the X series uprights.