well the two best mods at the moment for RTW are Europa Barbarorum (https://www.europabarbarorum.com) and Rome: Total Realism (http://www.rometotalrealism.org) both of which add much to the game but in different ways... another good mod is Rome: Total Gameplay (http://www.twcenter.net/forums/forumdisplay.php?f=118) if you don't want as much as the other two changes or SPQR (http://www.twcenter.net/forums/forumdisplay.php?f=32) which is a more fast paced game
but all in all the type of game you choose is up to you

to make sure you keep your previous install of Rome and have the ability to play the mod, I suggest using JSGME (https://forums.totalwar.org/vb/showthread.php?t=58201) which is a simple to use program…

hope I could help
GMM