I think you are certainly right that declaring war will be more difficult this time around. We already know that you cannot cross another factions borders without a military access treaty or a declaration of war, and that alone will make diplomacy more worthwhile.

As for recruiting foreign nationals, it may be possible to 'lend' and ally units to help in a war. Gifting units for a certain time period would be along the same lines as financial tribute, except much more useful for an ally. Ohter than that we might see foreign soldiers in the form of mercenary units.

It would be fantastic if nationalistic loyalty played a part in making it difficult to hold certain cities. For example if France took London they should face massive rebellion and popular uprising no matter what they do. Likewise European nations should not be able to wade merrily through Indian cities without seeing some backlash from the locals.

Another feature I would like to see is alliances forged between more than two partners. A league of nations formed by certain factions getting together for mutual benefit would go a long way to stopping one faction (usually the player) from steamrolling everyone else off the map.