Your question leaves me with an endless scope for possiblity in unanswering.
How does one play? Controls,plot, game modes, maps instead of asking for clarification I shall answer all, i well help you to understand the concept.
Controls:
Keyboard controls same as an fps; wsad make you move, mouse makes youn turn, left and right click for attack however this time it will take the melee model with different 'attacks' being applied to each click, space however is not jump; it is block with shield, instead of parry with sword which would require far too much skill, timing etc.
Plot and Game modes:
The difficulty here is that the Romans always fought in formation well clearly this is too boring and complex for a free to play multiplayer also it would require immense maps. Therefore I would suggest that the game was set in a time of turmoil in rome, civil wars etc where the was street fighting, this requires smaller maps and gives more freedom. Now this brings us to another problem historical accuracy of course I think this is not gonna be possible. But what I would suggest is that maps are authentic and accurate and that the only un historical thing is the mixing of weapons and the plot lines.
Game modes; I wold go along with the established model of team deathmatch, search and destroy, some form of monster vs man. The only one that needs addressing here is s & d as the romans had no bombs. Instead I would suggest firebrands to thatch.
Maps:
Urban areas from accross the empire. North african mud huts, to celtic mudhuts, to Gallic mudhuts and beyond!
Hope this answers your questions
Regards
Mastrannus