From my limited vantage point, I can already see some possible developments for passive leverage gathering.

Ancient Era

Trade routes: A simple way to start would be generating leverage for each foreign territory a trade route passes through. I am not aware of leverage on water tiles, so correct me if I'm wrong, but if that's the case, this could also incentivize land routes that pass through many territories. The natural limit on this would be the tenuous nature of trade, often disrupted by demands as it is, and a new wariness toward permitting trade.

Medieval Era

Medina quarter/caravanserai/funduq: This new quarter would help with urbanization and trade at the expense of new vulnerabilities. Adjacency FIMS yields, additional population slots, and leverage from trade routes to or through the city would balance out increased leverage gained by the owners of those trade routes. These could be limited to the Medieval Era and yield more leverage to foreign trade with subsequent eras. I am open to candidates from other cultures, but these came to mind as emblematic of the ease of infiltration that trade can present.

Industrial Era

Consulates: This would be a district built in non-capital cities of other empires, pending the appropriate treaty. They would have yields associated with consular services, facilitating trade, and promoting cultural/scientific ties. In exchange, they would provide the owner with leverage for each adjacent territory and could even serve as a spawn point for envoys and spies, though not sure how that would work! In addition to placing them in other empires, one could also build consulates in vassals' and independent peoples' territory. There would probably need to be a new leverage option to expel diplomats, which would destroy the consulates.

Contemporary Era

Intelligence agency/spy satellite: In the late-game, each power could build an intelligence agency to expand their leverage collection, give an espionage buff to their consulates, and most likely promote the agent class. A new spy satellite project could yield leverage for every territory across a band of latitude, with increased leverage given the worse diplomatic relations are.

Now, none of this is intended to address active components of leverage or how one would go about using all this extra leverage. Additionally, from the looks of it, era star requirements would probably have to increase to accommodate these extra sources. The core idea here would be to balance profitable yields from trade and diplomacy with the risk of access. It could also free up agents to circulate further afield as first trade routes and then cities become more reliable sources of leverage.

Looking forward to the expansion and excited by all the cool ideas these new mechanics will surely inspire!