Usually I like letting things play out, but sometimes I find myself purposely supporting a particular faction just so I can fight them late in the game when all the techs and units are in play. There are times when an old fashioned great game with a single rival faction throughout a campaign is very tempting. So how about this: what if there was an option to give particular factions handicaps to make them more powerful in the campaign. Say you start a new campaign as...Poland and you want a challenging Northern War sort of campaign but want to keep Sweden in play and make it a major rival throughout the game, with foreign colonies and such. To do this, just give Sweden a handicap before you begin your game.
Because it's optional, I think it would be an okay idea for both the Sandboxer and the player who wants to let things play out at random. Like I said, I'm a little bit of both. Historical rivalries are great to relive Britain vs. France, Prussia vs. Poland, Sweden vs. Russia, Spain vs. Netherlands etc.