Well, I still think we need spawning to best represent the 'hordes' coming across the table, but am still not entirely happy with the current mechanics; so wanted "something else" though this is really a variant on the original 9.2.1 rules.
To me the best description of Tyranids has always been the idea that there are 'unknown' numbers of Tyranids around (underground, off table or just out of sight of the enemy); you know there are lots, but do not have a clear picture of what 'lots' actually is.
Obviously the Tyranids work by attrition, throwing themselves against the enemy and trying to swamp them with numbers. However if the opponents can isolate where the 'heart' of the Tyranids army is, they can cut off the reinforcements being thrown in, and thus can swing the battle their way; by defining the numbers of Tyranids in the immediate vicinity (the table top part of the whole battlefield), the opponent can defeat the Tyranids strategy.
So in summary it is a battle over Tyranid reinforcements. Reinforcements appear from 'unknown' points in the rear of the Tyranid battlefield area and are fed forward by passing them from swarm to swarm.
To present this effect,
- Determine the number of reinforcements from the number of available spawning points as usual.
- These can only be added to swarms outside 30 cm *** from an enemy unit during the rally phase or using a marshal.
- Reinforcements may not be placed such that they can assault an enemy unit.
- If a swarm 'marshals', apart from removing BMs, the appropriate number of new Tyranid units may be added to the swarm that is furthest from any enemy unit.
- Tyranid units may be swapped from one formation to another during a turn (but doing so ends the activation of the swarm to which they are now attached), or in the Rally phase.
- Tyranid 'brood' units that are not under the control of a swarm during the turn may do nothing and are removed after the spawning phase of rallying.
- Reinforcement units must first be drawn from the dead units in the army list, but if none remain the player may introduce any other Tyranid units in his collection.
- If the battle goes to a point draw, this counts as a Tyranid victory (because their opponent has failed to find and stifle the heart of the Tyranid army).
*** a stronger, but probably overpowered variant is for units to be added to those swarms which have more than half the swarm outside assault range of enemy units.