I can see the insertion of a house rule for those bizarre alliance that don't seem to fit (orc/SM and the like). Some alliance fit "enemy of my enemy" in which the end of the alliance is civil. Necrons and Imperial Guard would be an example. The battle ends, both enemies walk away, warily. Alternatively, some alliances feature two forces in which one is using the other or one force is particularly bloodthirsty in combat or one force is using the cover of another to get things done. This third example would allow forces like space marines or necrons or eldar or dark eldar or tau to ally with tyranids.
Your main space marine force brings a detachment of tyranids. The fluff says these bugs represent a portion of a larger force overtaking the planet. Your main force is deployed ahead of the tyranid vanguard to evacuate a high ranking official on the planet who is being held by the chaos marines already there prior to the invasion.
The special rule would be "beginning in turn 2, any time a detachment squad consolidates (be it after regrouping or assault victory) and there are no enemy units in 12" make a leadership test. If the squad fails, they notice their nearest "allies" and hunt them out. They must move as fast as possible and assault as soon as possible. They are no longer under the owning players direct control. The squad must test for this each turn until they destroy their prey or their prey is more than 24" away at the time of the test."
Its not the easiest rule to read and I imagine can be written easier but this would mean that, if you brought an ally who fits into this niche (tyranids, orks, demons, based on who the main army is) you run the risk of losing control over them in a "fluffy fashion". Its also possible to play your army in a way that would avoid this. It might be a decent house rule to allow Tyranids to ally without actually being allies.
~600 points of Necrons - Phase 1
1500 points of Imperial Fists - Pictures Coming Soon
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