What I find interesting is that we can see Welles acting in Peter Brook's KING LEAR for CBS TV, which was followed by Welles directing his own production of KING LEAR on stage at City Center.
Of course, Micheál mac Liammór appearing as Poor Tom (Edgar) in the TV version is presumably because of Welles influence on the production... but one never knows for sure.
However, Peter Brook went on to direct Paul Scofield as Lear, who even Welles would certainly agree gave a brilliant performance, that he would be hard pressed to surpass. In 1966 Welles acted with Scofield in a MAN FOR ALL SEASONS, and the next year Welles wanted to cast Scofield in a movie version of JULIUS CAESAR, as Brutus against Christopher Plummer as Marc Antony, with Welles playing Caesar. One can only imagine what a film that would have made in the summer of love... "Welles envisions the Roman mob as hippies in the Haight-Ashbury, incited to violence after the assassination of Caesar!"
However, when CAESAR didn't work out, Scofield appeared in Peter Brook's movie version of KING LEAR, and Welles must have admired that performance tremendously, if perhaps not so much the actual direction...
I've no doubt, as a director, Welles felt he could do the play more justice. So, if Welles had ever gotten to do his own film version of KING LEAR, we might have seen Welles final Shakespearian masterpiece, even if he probably wouldn't have reached the heights Paul Scofield attained as an actor in playing the Mad King.