A week or two ago I went through the Orcs & Goblins army book and made three lists based on what I view as the main Orc & Goblin tribes. The first list is made up of Forest Goblins and Savage Orcs. This includes the Arachnarok spider and every other Spider-riding unit in the list.
The second list is Night Goblins and Squigs. I added some Stone Trolls to this list for muscle, as I figure they can be found in the same caves the Night Goblins inhabit. I also added a couple of Spear Chukkas, which aren't specifically Night Goblin war machines, but could easily be converted and probably would be useful for cave fighting due to having a flat firing trajectory when compared to Rock Lobbas and Doom Diver Catapults. I guess that doesn't matter too much in game terms, but it makes sense when considering the 'reality' of Night Goblins battling the Dwarves and Skaven for control of the underground. My suspension of disbelief only goes so far.
The third list is a bit larger, and is what you could call a traditional Orc & Goblin list. It's got all of the standard units (Orc Boyz, Goblins, Snotlings, Boar Cavalry, Wolf Cavalry, Chariots, Trolls, and Giants) as well as the war machines that didn't make the other lists (Doom Diver Catapult, Rock Lobba, Snotling Pump Wagon). It even has an Orc Warboss on a Wyvern. I wasn't sure about this one, as it's a huge number of points to spend on a single character, but I have the model and need an excuse to paint it up, so it's in the list.
If I were able to up my painting production and paint a couple hundred models a year, this group of lists would probably take me about 2 1/2 years to finish. That's a pretty long time, but at least now I have an idea of what I actually want to paint up for each army. I'm pretty sure I have some surplus figures that I can try to sell or trade for the things I still lack. I'm planning on making additional lists for every army I'm working on.