Tempted to give it the flagbearer mechanic but the onboard complexity is already really high in this set. Thinking about reducing the overall number of Instants and creatures with flash to offset it a little bit, which is an issue since White usually has a fair amount of Instants. Might make a lot of Assassinate-esque spells and Sorcery speed Reprisals to round out White's removal suite instead of the standard Rebukes and Gideon's Reproaches

Also I am like 95% sure this works.

Other issue: What races other than Human fit in the archetypical Roman caricature? Elephants? Elves? Hound people?

Barbarian slaves could be covered by orc/goblin maybe?

You've got the greeks - slightly foreign, respected (but conquered anyway) and considered wise. Elves seem to fit there.

Possible creature types:

Politicans: Vedalken (anything from Azorius), Rats, Elves.

Possible creatures for massed ranks of legions: zombies (would be a big reinterpretation, but they'd fit the "we obey orders better than anyone else" idea lots), goblin (not so much in recent interpretations, but very legion-y in some older ones), leonin (lawful warriors)

Animals: Elephants, Lions, Wolves, Dogs, Bears, Boars

Doesn't this have potential layering issues with pumps and whatnot? At least it raises some questions... Pendelhaven / Pendelhaven Elder are one time temporal effects so you just check the p/t once which is simple enough.

Nope, it works within the rules. Base power and toughness is set in Layer 7A if the card has a CDA defining P/T (and is default on cards that don't have CDAs defining P/T). In Layer 7B you get Shame and other P/T setting effects which this also applies towards. This effect applies on Layer 7C, which is the domain of pumps and anthems. Anything that gives a creature + or - something applies here. So unless I'm completely missing something, this is okay.

The potential rules iffyness probably stems from the lack of this sort of effect due to confusion potential (before they used the term "base power and toughness"). Now that we have that terminology, I think it's fine to make effects in this way since it's clearly spelled out as "base power and toughness".