They're adamant that both Infinity Ward and Treyarch will be onboard. The rumours spread from an ex-Activision employee of Underground Development which is scheduled to close.
That 'source' also claimed that UD was working on a third-person game for the Call of Duty franchise but the re-negotiation launched by Infinity Ward lead to their project getting canned and then presumably to their studio's collapse?
Call of Duty 4 has proven the most successful within the series and was developed by Infinity Ward; it still leads the charge in the UK's gaming charts.
