If I had to give my input on how I think things will work out, I think that some of the 15 Legendary Bladers will just be new characters to fill in the remaining numbers much like how MFB had the rest of the Legend Bladers (Outside of Ginga, Kyoya, Ryuga and Kenta) as completely new characters.
Of course, it is possible that some more returning characters will come back but realistically, the only other ones I can see potentially returning are Xhaka and Delta.
Yes, I know Xhaka retired in Cho-Z but you could make the explanation that with the increase of much stronger bladers, he became interested enough to take Xcalibur out of retirement. The Xcalibur line has been one of the longest running lines in the Burst series (only being absent in GT). If not Xhaka, then at least Xhan.
Delta's another character I see potentially coming back since besides his popularity and how the anime pushes him out as a breakout character, Drum and Delta have yet to properly settle their rivalry and since we only have one GT representative, it would make sense if they bring out another one. There's Amane too, but I kinda doubt he'll be brought back since we've got back our resident test dummies for advertising new beys, the Kiyama brothers.
Eh, I will think of something to put here...
... later ;)