I have often wondered about this, there doesn’t seem to be much idea of what an Early Imperial Roman army would have had as a standard. My conclusion was ‘why would they even need one?’ The Standards were Rally Points and Sacred objects for the formations involved. Would a Legate need one? He would either be fighting as part of a Legion if he was a Caesar or Marius type, or at the back watching his Legion carry out his plan while eating Aardvark toes on toast if he was more of a Lucius Licinnius Lucullus or a Pompeius kinda guy. If you feel the urge, a Vexillum would do no harm, would not look out of place. If your lads are wearing Banded Mail, there will have been no consulars around with Lictors, Augustus made damn sure of that 🙂
I am fairly sure that the commander would mark his presence on the battlefield so that he made his presence known to the Legion commanders. Caesar at Pharsalus, for example, positioned himself behind the right (open) flank of his legions. This was where he decided that the critical ground lay and had concealed a reserve force to counter Pompey’s expected flanking cavalry attack. He wanted to send the concealed troops into action on his command at the critical moment.
My legionaries are the Warlord games plastics wearing lorica segmentata plate armour.