I'd been thinking about the same thing, but got distracted before I had enough of a list put together to suggest it.
Here's what I've got together so far. Some of the dates are a little off for the starting time of the mod, if that's a problem. Also, some of the current nobles' names are a little vague, so I don't know if some of these are repeats or not. They're taken mostly from my copies of Froissart, but I think I'd taken some from my Chandos' Herald and some from a handful of secondary texts on the HYW.
English
Sir John Chandos (d 1370)
Walter de Manny (d 1372)
Joanna of Flanders, Duchess Montfort (d 1374) or her son, John Montfort (d 1399)
Jean III de Grailly, Captal de Buch (d 1377)
Sir John Hawkwood (left for Italy in 1363, routier)
Sir Robert Knollys (d 1407, routier)
John of Gaunt, Duke of Lancaster
Geoffrey d'Harcourt (d 1356)
Thomas de Beauchamp, Earl Warwick (d 1401)
Thomas de Vere, Earl Oxford (d 1371) or his son, Robert de Vere
William d'Ufford, Earl Suffolk (d 1382)
William Montacute, Earl Salisbury (d 1397)
Lord Thomas Percy, Earl of Worcester (d 1403)
Sir Hugh Calveley (d 1393)
Thomas of Buckingham (d 1397)
Richard FitzAlan, Earl Arundel (d 1397)
Humphrey de Bohun, Earl Northampton, Earl Hereford, and Earl Essex (d 1373)
Sir Reginald Cobham (d 1361), or his son, also Reginald Cobham
Sir Thomas Holland, Earl of Kent (d 1360)
Sir Louis de Beauchamp
Sir James Audley (d 1369)
Sir Bartholomew Burghersh (d 1369)
John, Lord Mowbray (d 136

Admiral John Neville (d 138

Henry Percy, Earl of Northumberland (d 140

Ralph, Baron of Stratford (d 1372)
Thomas de Mowbray, Duke of Norfolk (d 1399)
French
Enguerrand de Coucy (d 1397)
Arnaud d'Albret (d 1401)
Louis, Duc d'Anjou (d 1384)
John, Duc de Berry (d 1416)
Philippe II le Hardi, Duc de Burgundy (d 1404)
Louis, Comte de Flanders (d 1384)
Louis, Duc de Bourbon (d 1410)
Admiral Jean de Vienne (d 1396)
Olivier de Clisson (d 1407)
Guy d'Asai, Viscomte de Narbonne
Marshal Arnoul d'Audrehem (d 1370)
The Archpriest, Arnaut de Cervole (d 1366, routier)
Geoffrey de Charny (d 1356)
Gaston Phoebus, Comte de Foix (d 1391)
Robert, Duc de Bar (d 1411)
Most of my reading has been in the earlier stages of the war, and many of the earlier leaders were dying off by 1371. Sorry.
Ajax
ps. Many of those same lords in need of new names could also use some new gear as well, and maybe even some new faces.