I have some thoughts on some traits / definitions / special rules that might be necessary for other periods. I'm mostly doing this because I don't have the armies for Aurelian as published and I want to push the figs I do have from other periods.
But before I get into that, I want to first say that I am a big fan of elegance and simplicity in rules design. Sam is masterful in this respect. So I think that limiting traits and unique rules for units/periods should be a guiding principle.
Having said that, I can think of several unit types that *might* require some special treatment.
I also don't know how far Sam would want Aurelian "stretched". I'm going to talk some into the high medieval age, and Sam might want to do a major supplement or replacement of Aurelian before that point in history. So forgive me if I'm kind of making a long pass, here.
1) Phalanxes - Frontal assaults on true phalanx type infantry was pretty difficult if they were in good order. In my opinion, true phalanxes include classical Greek hoplites, Macedonian/Successor phalangites, and probably Swiss pikemen. For these types of units, I think maybe a "Phalanx Trait" might be appropriate. It would halve (FRU) the Úlan of enemy cavalry attacking the front of the unit, if the unit is in open terrain and has two or fewer disruptions. I also think we should be careful about what units are "phalanxes". There are many spear-armed units in history who should not receive this trait. Only those which demonstrated the discipline and order to defeat frontal cavalry charges should get it. One of my gripes for other rules is treating all spearmen like they are hoplites.
2) Pikes and Spears - Just as an aside, I think these require some careful consideration for their Úlan. I personally think average spearmen should at best have an Úlan of 4. Elites an Úlan of 5. (I'm using the Romans as base of reference.) Average pikemen should probably have an Úlan of 5, with elites (think Hypaspists or Swiss pikemen) having an Úlan of 6. I could also see pike blocks taking more than five hits to break -- like maybe six.
3) Chariots - I'm not even sure what should be done here. But something will be needed. You can't really treat them like cavalry on wheels. I'm not going to hazard suggestions -- I just can't think of good ways to handle them.
3) Knights - My guess is that the elite high medieval knight might have an Úlan of about 7 with four armor. Just guessing. These units might stretch the game design, or might be easily accommodated.
4) Longbow - I generally don't like the idea of weapon-specific considerations -- but longbows would require something. Probably a special trait that raises their effectiveness vs mounted targets.
5) Cav armies -- my favorite period is dark age. I don't think the core army concept is appropriate for this period for some of the armies (Sassanids, Byzantines, Vandals). Might drop it -- or have it apply to cavalry units. Not sure.
This is mostly thinking out loud. Not meant to stir up too much controversy.