As much as I’ve been touting greater use of emergence in games, it’s a non-trivial problem in terms of execution.  Developers face a few challenges if they go down this path.

Balance

The massive possibility space that’s so beneficial to player engagement also happens to be impossible to balance.  It’s simply too large to test all the permutations.

Talent, experience and iteration go a long way toward dealing with this issue, but a simpler approach is to re-think what it means to be balanced.  If unusual, lop-sided situations are re-cast as part of the experience — and the build-try-fail loop is short — players will find them entertaining instead of frustrating.

Being a live product also helps– anything too extreme can be quickly adjusted if it’s having a large impact on play (and by large impact, I mean it’s driving all players to pursue a single strategy, thereby undermining the benefits of an emergent system).

Noise

Having a large possibility space isn’t much good if it’s hard to find the interesting bits.  Similarly, if players can’t differentiate one approach from another, then being emergent isn’t going to increase engagement.

In both cases it helps to make the core elements of the game — the ones that can be combined during play to produce different results — as orthogonal as possible.  Having a +1 sword, +2 sword, and +3 sword as options is not being orthogonal.  A better approach would be attributes that are more binary in nature:  sword vs. not-sword, AOE vs. not-AOE, DOT vs. not-DOT, ranged vs. not-ranged, etc.   The ability to combine any of those together creates a much more discrete, understandable and useful set of permutations for the player.

Representation and Visual Fidelity

Emergence works by combining core elements in many different ways, but in a large possibility space it’s not possible to create unique assets for all permutations.  Emergent elements are often represented by simply visually stitching together smaller bits.  The challenge is making the combined element quickly understandable, even if the player has never seen that particular combination before.

Having only a handful of core, orthogonal elements helps, since the player only has to learn their function and not all the permutations, as does limiting the number of elements in any combination.  Really strong art direction makes a big difference too, particularly on smaller gaming devices where core elements might be very small.

Take poker as an example.  The individual cards are core elements, and a hand is an emergent element.  The uniqueness of each hand comes from stitching together a few cards, not a new asset.  The number of cards in a hand is constrained, each card is visually distinct, and the number of possible hands is very large.  But the player can immediately recognize what they have, even if that particular set of cards is new to them.