It seems that MPEG-1 Layer 3 patents are mostly expired, the core patents for MPEG-2 will expire in 2016-2018, but MPEG-4 MPEGLA patents are safe till 2025-2027. Also if the patents expire the organizations loses it's ability to collect royalties for the codec it self from what i understand so if that would've been the case there's little need to create a new codec but MPEG4/AVC patent protection seem to remain quite strong for about another decade.

The MPEG4 licensing scheme is kinda confusing to me it seems that several organizations collect royalties individually even tho it states that MPEGLA collects them in the US, AT&T seems to do so too for their share of the patents.