Hello!

In unmodded factorio, attacks are being sent out based on how much pollution a base is consuming, with each biter/spitter having a certain pollution "cost" (regardless of evo, but higher evo required to "buy" the higher biters/spitters).

With this information established, having this mod integrate the pollution mechanic would be very nice.

For example, a small biter costs 4 pollution. A small biter with 1 extra life could cost 6 or 8 pollution. This allows the mod to tie into the pollution mechanic nicely. There are pros and cons to these; a biter that revives upon death 1 time costing 8 pollution tends to be less powerful than two biters without revive at 4 cost (same final buy value) unless there's AOE involved; then a biter reviving is more effective than two biters dying from 1 shot.

Another benefit to having it integrate with the pollution mechanic is that it will have an impact on attack size and thus a positive impact on UPS performance.

Extrapolate:
Having a Behemoth biter with a total of 10 lives cost the same pollution as 10 behemoth biters with 1 life would be a great improvement to UPS, while keeping the pros & cons of "weak in low number but AOE resistant".

You could tweak the pollution cost so that it's not exactly "8000 pollution = 20 behemoth biters with 1 life OR 10 behemoth biter with 2 lives" depending on how you want your balance to be.

You might be concerned with mod compatibility or code requirement to make this work (i.e. you'd have to pre-determine how many lives a biter gets upon spawning), though it would be the neatest to do it the way I described above, there are alternative ways to do it, such as having a revival trigger the pollution scrubbing event instead. This would cause pollution to lower in the cell it respawned in instead of at the base spawn (which is slightly less powerful I believe because tiles reduce pollution by a percentage and would mean "passive scrubbing" is strongest near the source. So a behemoth biter reviving near the base scrubbing 400 pollution will have a lower pollution (less balanced, in the enemy faction's favor) reducing impact than if it was done upon biter spawning at the nest.

Anyway, some food for thought.

It's an interesting idea.

There is another approach which keeps the pollution cost at the spawner end, but avoids hard coding pollution cost and maintains variation in revives per individual biter. When a unit is added to a unit-group an event is triggered, so this would be triggering as the attack group is accumulating. As the pollution payment is made when existing child biters of a spawner are added to an attack group forming.
I could pre-determine the revival count of this individual biter within the current revive chances and then subtract this additional pollution from the spawner/chunk. I'd cache this revive count against the biter and then when it dies revive it this number of times.
This would keep the variation in revives per biter, but would lock in the revive count at attack group formation point and not at death point. So the effective revive chance per biter when it dies would lag the current conceptual revive chance by a bit.

And yes if done under whichever mechanic the pollution cost for a revive would need to be thought out. I suspect a partial cost of say 50-75% would be appropriate. But I'd likely make it a mod option. As you point out the weapons used to defend will affect an appropriate cost. As late game may be flamers with intricate walls, or just massed lasers.

As a further thought if it was done from pollution in chunk the biter was revived at it could be dependent upon the pollution in the chunk the biter was revived at. Hmm many options.