Recently and not-so recently I've written about casual games, and how "simplicity" isn't something you can assume for much longer.
Casual games are a fairly new market, and are just now developing the power of genre. Looking back at early computer games and arcade games, we see the same basic thing happening: start simple, get complex.
Each "wave" of game has addressed a new audience. First, video arcades. Their games were short and hard, meant to eat quarters and keep players churning together. Second, home systems (computers, Atari, etc). Their games were intended to convince players that the $60 they spent was worth it by doing just the opposite: taking forever to complete. Third: casual games. Addressing an audience who will often play for a long time, but only in small chunks and games which can be interrupted and canceled.
Each wave addressed its audience in very particular ways. Mechanics appropriate to one group need much tweaking to be appropriate to another. But each wave, without fail, steadily added complexity.
As genres were built up, short-term complexity was added. More moves, more items, more buttons, more options. Each built on the standards that the audience had liked in its genre.
You can already see this happening with Casual Games. Atlantis Sky Patrol, Virtual Villagers, Fish Tycoon, Diner Dash 2... all of these games might not feel complex to you, but compared to their ancestors, they are as much more complex as eels to earthworms. This trend will continue, restricted by the nature of the audience: an audience that can't guarantee even five minutes before the boss walks by. An audience which requires interesting play literally every minute of the game.
In addition, however, as genres were built up, long-term complexity was added. No matter what the target audience, games began to keep data long-term. First, it was simply top scores. Then it was characters and positions. Now, whole rosters. This is true of home systems, this is true of arcades, and it is coming true of casual games.
But the same restrictions apply. The long-term complexity that will be added to casual games will be, at least at first, a mild sort. Think those 3D chat avatars, or the ability to build and furnish your own homes, or to put together a complex friend tree. But as closely related to the casual games as a first-person-shooter dossiere to the first-person-shooter.
Obviously, the way this is most likely to succeed is if some game place, something like Popcap, creates a piece of middleware. The backbone of the middleware creates a massive, shared system for avatar customization, stuff collecting, friend relations, permissions, and so on. Then people can create games - mostly casual ones - which feed back onto that backbone. Like Secondlife, but without the horrible pain.
This has a whole bunch of interesting features. For example, you'll have to arrange it so that pornographic content can't be popped up on your screen without permission, ever. These are often people at work and/or children. Also, older games which few people play will still have unique rewards to offer, so there will be steady "trawling" for "oldschool loot"...
We're likely to see this kind of backbone driven by micropayments, but exactly how that will be set up is still a mystery to me. There are a lot of options.
The effect of this scattered-attention audience on "narrative"-based games is pretty dramatic. The narratives we see on big games of today won't be happening. We're more likely to see very slow narratives (adventure-title type narratives) and more of a "legend-style" narrative, where it's more about character design and a few critical events rather than a specific progression.
There are other effects... it's an exciting field!