Another option is to not do research, somewhat freezing your imperium at a desired level while you build a strong enough base from which to expand.

Both are a gamble as his heir could have the same trait, and going to war to kill a general in counter productive in the short term.

You can deal with this guy by using an agent to assassinate him or simply find and kill him on the battlefield.

Raising Imperium can have a very negative effect on diplomacy if the rival faction leader has the trait 'Hates Rival Empires' with the subtext that he hates any faction who's impearialsm threatens his expansion.

No Distinction: if a leader has no reference to an empire he will have -30 ' Great Power'.

Admires Strong Empirs: This guy will have +30 ' Great Power'.

Indifferent to Rival Empires: For this guy ' Great Power' will likely be a single digit.

Hates Rival Empires: This guy will have -75 on his view of you denoted as ' Great Power' in the list.

There are four important distinctions in traits. Leaders of rival factions can view you in drastically different ways based off their personal traits, visible in the diplomacy window. Imperium will have a very heavy effect of diplomacy. I just had the guy killed and moved on, oops. Unfortunately I failed to verify if this was related to a trait, and what trait. Imperium can effect the loyalty of your characters (generals, statesmen, etc). Governors are likely the most important facet of Imperium in regards to your faction.

Imperium effects the number of armies, fleets, agents, and governors you can have.

Total war attila imperium