So, a couple months ago, before Empires was announced, I was wanting to mix things up a bit, so I came up with the idea of "realms." These are fixed effects, chosen as part of the Kingdom selection, that affect all players (for example, a realm that said, "At the start of each of your turns, +1 Card," would have the same effect as each player having a Hireling in play, barring edge cases like Peddler). Some of them do nothing, so you still get some normal games. Note that a couple of them do use Coin tokens or Dark Ages cards (shelters and ruins).
There are two types of Realms: Government, and Terrain. They are mixed together in a deck, and you draw until you've drawn one of each. Keep the first Government Realm, and all Terrain Realms (except superfluous Grasslands; see below). Put the selected Realms next to the Supply.
Of course, you can tweak the setup rules and the number of "No effect" Realms to suit your tastes.
Terrains:
Grasslands (4 copies): No effect.
Note: As noted, I'd recommend not using this if you also drew another Terrain card (and not using more than one Grasslands if you drew multiple copies). If your only Terrain card is a single Grasslands, it's nice to have it to show that there's no Terrain effect, but otherwise, it's just superfluous.
Fertile Plains: At the start of each of your turns, +1 Action.
Coastlands: At the start of each of your turns, +1 Buy.
Note: I started with the basics. I didn't include +1 Card or +$1 Realms because I didn't think that would change the gameplay as much as automatically getting an extra Action or Buy each turn (having 6-card starting hands isn't that much different from having 5-card starting hands, and there are plenty of ways to generate income).
Great River: When you gain a card, you may put it anywhere in your deck. If you would put a gained card on top of your deck (except by this ability), you may instead set it aside. If you do, put it in your hand at the start of your next turn.
Rocky Highlands: Victory cards are worth 1VP more. Provinces and Colonies cost $2 more, and all other Victory cards cost $1 more (prior to any cost reductions).
Note: The reason Provinces and Colonies have a bigger cost increase than other Victory cards is to reduce the adverse effect on alt-VP.
Vast Steppes: Each non-Kingdom Victory pile has 50% more cards than normal.
Note: Yes, this requires both the base game and Intrigue (or the Base Cards set) for multiplayer, but I assume most players have that. In a perfect world, it would also affect Kingdom Victory cards, but that wouldn't work in multiplayer (unless you had multiple copies of the same set, or used placeholders).
Groves: At the start of the game, each player places a Coin token on each Kingdom Supply pile. The first time you gain a card from a pile, take your Coin token from it.
Note: A little reward for variety. The reason it only effects Kingdom cards is because it
Dense Woodlands: After resolving each Action and at the start of your Buy phase, you may discard a card from your hand. If you do, choose one: +1 Card, +1 Action, +1 Buy, or + $1. You may choose each bonus only once per turn.
Note: You have to select a bonus if you can (so if, for some reason, you want to discard a fourth card when you've already selected +1 Action, +1 Buy, and +$1, you have to take the +1 Card, even if you'd rather not draw a card). However, after you've selected all of the bonuses, you can still discard cards if you want (although Dense Woodlands doesn't give you anything for it). I tried a version that set aside cards and discarded them during the Clean Up phase to avoid triggering Tunnel, but I'm not sure that combo is so overpowered to require the additional complexity (compare, say, Market Square-Bonfire or Market Square-Plan).
Rich Valley: At the start of the game and the first time you gain a Victory card, take 3 Coin tokens.
Note: A nice boost to the opening, and another when you start greening. You could purchase an early Estate just to get the Coin tokens sooner, but unless you have a specific plan for it, I don't think it's generally worth it.
Governments:
Feudal Kingdom (8 copies): No effect.
Fallen Empire: Replace your starting Coppers with 1 Estate, 1 Curse, 2 Ruins of your choice, and 3 Spoils. You may choose the order of your starting deck (prior to drawing your first hand).
Note: Make sure to buy some Treasure (or virtual coin, gainers, etc.) or you could be stuck buying Copper. I let you choose the order of your starting deck because the difference between a 6-3 split and a 9-0 split is often too great to leave it up to luck (or maybe I'm just risk-averse; to be honest, I already normally play with a house rule that you can choose the order of your starting deck).
Ancient Kingdom: At the start of each of your turns, you may trash a card from your hand; if you do, +1 Card per [coin] it costs. You may buy or gain cards in the trash as though they were in the Supply.
Note: The initial idea was to buy or gain cards from the trash, which meant I needed an incentive to trash decent cards. In the end, the Apprentice ability overshadowed the gain-from-trash ability, but it's turned out to be fun enough that I kept it.
Noble Cities: The first player starts with a Curse as an additional starting card. At the start of each of your turns, +2 Cards, then put a card from your hand in the discard pile of the player to your left.
Note: This was my brother's idea. Like Masquerade, but without the trashing (and it gives you +2 Cards for the same reason as Masquerade). I added the starting Curse because I thought it would be fun to have an "Old Maid" to pass around in games without junkers. Note that the Curse doesn't come from the Supply; you still have (n-1)*10 Curses in the Supply in an n-player game (just like how starting Estates don't come from the Supply).
Uninhabited Lands: Start the game with no starting cards and 5 Coin tokens (prior to any setup effects). Include Platinum and Colonies in the Supply. When you would trash a card during your turn, you may instead gain a Copper; if you do, +$1.
Note: This is the oldest Realm; it started from an idea that it would be fun to play a game with a single Workshop as the only starting card, or with no starting cards and Alms in the Kingdom. The reason for including Platinum and Colonies is just to add to the "building an empire from nothing" feel. The trashing language is so that trashers aren't worthless. The reason you don't just gain the Copper to your hand (which would be simpler) is to keep it from being too easy to autopile Rats (which happened in one of our early games). The "during your turn" clause is to keep it from blocking trashing attacks. Note that if you play a mandatory trasher and don't have any eligible cards to trash, you can still choose between gaining a Copper and trying to trash a card (even if the latter ultimately fails). The same is true if there are no Coppers in the Supply (although you don't get +$1 unless you actually gain a Copper).