For video we already know that after H.264 there will be H.265 with the promise of 50% better compression & indeed higher complexity & higher minimum hardware requirement.

Is there already such a planned on paper successor for AAC by the MPEG consortium ?

Not too long ago HE-AAC and ALS (lossless) were added to the MPEG-4 standard.

They could come up with another format just about when the patents on AAC will be expiring. But does the user need another format? Today we can have the whole range of quality settings: from cellphone to transparent. Any new format will be incompatible with existing decoders. The CD-DA format is still in use, why couldn't be MP3 or Vorbis?

Spatial Audio is one of the latest MPEG things.