1) Continue to involve the PCs as individuals and incorporate their stats and abilities;
2) Be easy to learn but difficult to master; and
3) Have a wealth of expansion and customization options.
Well, just this month both 13th Age and D&D 5E have released mass combat rules. I'm interested in taking a look at them. 13th Age's rules are just in the prototype stage and seem to be much more story than mechanics-driven (which is not surprising given 13th Age's mechanics in general). I'll take a look and let y'all know what I think. If you have thoughts, post them in the comments.
13th Age Mass combat rules here.
5E Mass combat rules here.