I liked how in Napoleon the major countries had great historical generals that would not die but rather be wounded and return to the capital. Each of the major powers in WWI had a general that could be given this attribute. Here's a list of the ones I think would be the best.
UK: Douglas Haig
France: Ferdinand Foch
Germany: Erich Ludendorff
Russia: Alexei Brusilov or even Tsar Nicholas
Italy: Luigi Cadorna
Austria-Hungary: Conrad von Hotzendorf
Ottoman Empire: Mustafa Kemal (aka Ataturk)
Belgium: King Albert I
U.S.A: John J. Pershing (custom battles)