xion absorbed by roxas.
Roxas absorbed by Sora.
Vanitas absorbed by Ventus.
Ventus absorbed bysora.
Sora's heartless Brought back to human by kairi so sorta re absorbed into sora.
So this theory doesn't work because sora is already completely combined except for data sora but mickey wrote that letter telling him what he needed to know so data sora is also combined with real sora.
Them combined now doesn't rule out a de-combination later on as all these except Sora's heartless are in essence independent persons with an own mind.
Furthermore, out of all these, Ventus is almost guaranteed to be back at some point.