*ahem*
A secret ending doesn't mean it only has to tie into KHIII, you know. It could easily show something about Coded, or something that The Organization did after or before BBS. It could have a flash-back to the six apprentices. Besides, Xemnas's mysterious actions (talking to the armor) and the mere existence of a 14th member submit grand impact on KHIII.
As far as BBS secret endings go, I think it would be nice if the final scenes showed AtW discovering Xehanort and the original six performing experiments that create the heartless and their nobodies. Then, I think it would be nifty to see Maleficent taking over Radiant Garden and having AtW sending Kairi away to Destiny Islands. A nice little KHI set-up.