I've mentioned this in a different thread earlier today, surprised nobody else has brought up the name of Liam Brady.
An ideal candidate. He's experienced in dealing with young players at the Arsenal Academy. Has a great working relationship with Trap so Brady would know exactly what the Senior manager would be looking for in terms of players and tactics so the U21 team/tactics could finally act as real support to the Senior squad as U21 players promoted to the senior team would have plenty of experience with Traps tactical style. As opposed to just being abused from a height by Givens.
People might say that Brady would never take on the U21 job due to his commitments with Arsenal but he has ambitions to succeed Trap in a few years time in the big job. The U21 role would be great experience for him, better even than the role he had in the senior team. With any luck a handful of the current U21s would be able to progress into the senior team in three, four years, if Brady did come to be in charge of the Senior team a lot of players would be familiar with his management style.
In an ideal scenario i'd like to see Kenny Cunningham involved as well in an assistant role to Brady. He's eager to get into management and working with Brady would be an ideal start for him. If Brady was to move up that would leave Cunningham to continue on in the U21 role. Unlikely I know, but as I said, that would be my ideal scenario.