It is possible to play as any faction by changing the descr_stat.txt
file. However, this is not an official mod, so do this at your own risk.
This website will be helpful (if you're not confident in changing game files I wouldn't do it).
This will tell you how to change all the factions in the game into playable (all but rebels, as rebels = slave), even the main Roman Senate faction. Playing as Rebels/slaves or Roman Senate is buggy so not recommended for regular play.
The
descr_strat.txt
file can be found in thedata\world\maps\campaign\imperial_campaign
folder[...] [the below] section determines which factions are playable[...] To change this, simply cut and paste them into another section.
The section of descr_strat.txt
you're looking for looks like this:
playable
romans_julii
romans_brutii
romans_scipii
end
unlockable
egypt
seleucid
carthage
parthia
gauls
germans
britons
greek_cities
end
nonplayable
romans_senate
macedon
pontus
armenia
dacia
numidia
scythia
spain
thrace
slave
end
Bonus tip: You can look in the building browser of cities you do not control to see the units/buildings for that faction.