I've been thinking while working my way through the AD&D monster manual about how many monsters I would use if only they were lower in power. Obviously one can easily scale the monster down to a more reasonable level, but I wondered what a scalable statblock would look like.
Armor class could remain fairly constant, if you are assuming a low level system/setting, with a bounded armor class, so that wouldn't need to scale. The main things that would need to scale would be hit dice and attacks/damage, which you could decide to link together.
For scaling hit dice, I will use the notation "L" to represent the dungeon level. So a monster's hit dice could be written as L+1, indicating that their hit dice are equal to the dungeon level plus 1. Thus, a monster of that type encountered on the 3rd dungeon level would have 4 hit dice. This can also work in reverse, with a monster have L-2 hit dice. A monster whose hit dice would be considered 0 actually has 1/2 of a hit die, and a monster with hit dice in the negatives is treated as having a single hit point.
Special attacks are more difficult, but you could use the simple route of dragon breath's damage, having special attacks deal damage equal to the monster's current hit points.
So, as an example, here is a scalable statblock for a giant black widow spider
|Credit to DeanSpencerArt on deviantart|
Sorry for the kind of rambling post, I just got off work and HAD to write this down.