I was going to log off (and I did), but an idea popped into my head so I came back on.

I was thinking of how the rate of entry would still make it annoying to get a weaker creature instead of a stronger one as I was using the restroom, and a simple notion based ENTIRELY around your counts-per-creature idea came into my head. For simplicity's sake, let's say that there is a population cap (which I don't think there is), and that creatures come in three grades. I will define each of these.

First, the population cap is determined by portals and perhaps other factors, such as map-specific settings and a base creature cap. Second, minions come in three ranks: Weak, Average, and Strong. (*Note: These are not my suggested names or settings for game mechanics, merely filler.*) Weak creatures count as 0.5 population, Average as 1.0, and Strong as 2.0. VERY strong creatures may, perhaps, count higher than 2.0, but these will likely be rarer or be Hero units.

Now that we've decided these things, let's say that, in our scenario, we have a portal. Let's pretend that we currently have no creatures except imps (who do not count against our creature population limit) and that the time ALL creatures take, no matter what your population is, starts at a base time of 30 seconds. (*This, again, is not a suggestion, merely an example, and I find it much more plausible that there would be variation on the time based on your current population.*)

Now, let's assume that there is a formula for creature generation. With this formula, because of our scenario's specifications, we're ignoring the current population. So, the formula is this: xt=c, or rank * time = creature generation. Since we've asserted that 't' is 30 seconds, the only variable remaining is 'x,' the rank of our creature.

A weak creature takes 15 seconds to enter your dungeon, an average creature takes 30 seconds, and a strong creature takes 60 seconds. Simple and straightforward. What's just as important, here, is that the weak creature is 1/4 the strength of a strong creature (generally speaking) and also *takes 1/4 the amount of time to enter your dungeon*. In other words, the creature strength directly influences how long it takes for the creature to spawn.

In order for this to work, the creature type, or at least the creature rank, would have to be determined *as soon as your last creature was created*. I.E., the moment creature 1 steps out of the portal, the game decides what creature 2 will be; the moment creature 2 steps out of the portal, the game decides what creature 3 will be. (*Again, you can safely replace 'creature' with 'rank' and it will work just as well.*) If you are no longer able to recruit the same creature due to a lack of rooms (such as losing them to a rival Underlord), then the game can be designed to either A) let you recruit the creature anyway, or B) "reroll" what creature you get.

Again, this is just a slight deviation / addition on the idea Keith presented. It should also be noted that the actual formula for the creature appearance would look more like x(formula that determines base creature recruitment time based on population)=c. I know, that may be obvious, but I felt it important enough to write down anyway.

Anyway, that's my idea. Pretend I ended this sentence with something utterly cunning and hysterical, please.