I'd like to see a whole DLC devoted to overhauling empires, personally. Forming an empire should be a long, interesting, difficult quest, not just the click of a button. There should be more risk, more real player involvement, and ultimately more reward, in the sense of a richer, more detailed experience, with each empire being distinct from every other (using the exact same rules to recreate both the HRE and the ERE is, as many people have observed over the years, not the most flavorful/realistic/enjoyable thing in the game).
Spending all that gold and piety to get the empire title should just be the first step—there should then be an expensive, challenging, years- or decades-long process of setting up the empire's legal and administrative structure, with plenty of events, decisions, and opportunities for failure (genius emperor dies, incompetent son takes over, factions form, everything goes to shit). The process should be smoother under certain circumstances (e.g., Zoroastrian Persian ruler tries to restore Persian Empire) and rockier under others (trying to create an empire that's never existed on territory mostly inhabited by a foreign culture/infidel religion). And if things go wrong before the empire is fully, stably established (or if an empire loses too much territory, or if a faction precipitates a crisis), the title should be destroyed.
