On building a system.... i've been mucking around with a 40k rpg, and the best idea i have had so far is simply to list a number of books that should be read for ideas on what the setting is like, fill in the gaps a bit and then use the WHFRP system. Maybe tack on the =I= combat system. But apparently there is already one done by fans, so if i find that i would probably just note a list of things i would change and leave it at that.
For a historical system, just do the following...
1) List 2 or 3 good books for each era or civilisation. With all the historians on the board, that should not be difficult. Or even just say 'go buy a gurps book'.
2) Include some notes on conversion for game systems. Should be easy enough - use D20 NPC classes for everyone, use the human 6/4/3 profile for white wolf STer, use base system for warhammer roleplay, only no fate points etc.
Weapons should be fairly simple - most of em are alreaddy listed in most systems, and the others (eg, pilum) are generally variations (a pilum was, to my knowledge, just a really heavy javelin that was short ranged and used for multiple ranks of regiments to fight with).
Lose magic etc.
I agree it would not be different from an average RPG... i mean, consider the following.
'Discover the intentions of the orc horde threatening the human kingdom of Lath'mar, and guide the defenders to the best points to attack the orcs.'
changed to:
'Discover the intentions of the army of Boudiccea that is threatening the roman holdings in england, and guide the legions into battle'.
After all, Gladiator could be run as a one-PC game. It converts fairly well, just lose the battle at the start, or make it more manageable for a 1st level character.