I do a lot of brainstorming and daydreaming about 40K armies, and it's a constant source of torture for me.
I own thousands of points of unassembled and unpainted Space Marine models and parts. I also have my several thousand points worth of Ultramarines, painted and assembled.
The players at my FLGS know that I've recently been struggling with the idea of changing Chapters to something a little more exciting or obscure. I go back and forth on that idea repeatedly, and never do anything about it. The reasons are numerous.
Despite my self-knowledge that I'll never restart my existing army, or start a new one, I continue to think about the process of doing exactly that. I have a "game plan" for several armies that I'd build if I had limitless free time and income.
In an effort to alleviate some of my brain pain by writing things down, here's some of my more prevalent ideas:
Chaos Marines: The Horned Host. A warband of Tzeentchian Chaos Marines who have been cursed with mutation. Spines, horns, and quills erupt from their bodies, often bursting straight through their power armor. Pretty generic, but it would be an interesting project to modify all those mutations.
Chaos Marines: World Eaters warband. If you're gonna go "established Chaos fluff" you can't beat the World Eaters.
Dark Eldar: Coven of the Needle's Eye. This is an excuse to scratchbuild a horde of Wracks and Grotesques from various models. I have this nefarious plan to build Wracks from Empire Flagellants models. Sadly, this idea was largely squashed when Webway Portals became pointless for melee armies.
Imperial Guard: Catachan 27th Tank Company and Catachan 30th Artillery Company. The tank company would have used the Imperial Armor Armored Battlegroup rules from the now-defunct downloadable PDF. It would have to come from the newly updated IA:2 book now. The artillery company was a focus on troops units with sniper rifles and mortars, backed by as many artillery pieces as I could lay hands on.
Imperial Guard: Mordian 95th Regiment of Foot. Gunline. Nothing but a sea of out of production metal Mordians as far as the eye can see. SUPER expensive project.
Orks: Scrapgrabba's Loota Boyz. An Ork army led by a Big Mek (Scrapgrabba) with tons of converted, looted models. Lootas, Burna Boyz, Kans, Dreds, Boomwagons, etc. Basically a big mob of mek-smart Orks who build all their kit from scrap. I'd include as many random-effect units as I could. I feel dirty even thinking about making an Ork army.
Tyranids: A completely shooting-less army of gribblies. Masses of Genestealers, Hormagaunts, Raveners, Lictors, Carnifex and Warriors. No spawn-poopers or anything fancy. Just tons of melee dice.
Tyranids: Retro Second Edition army. Build an army using only the units that were available in the old second edition codex. I can't decide if I wanted to build this one using authentic second edition metals, or with current plastics. It would probably make more sense using the old metals.
Those are the sorts of things that bang around in my head every few days. So many cool projects to try, so little time!
On the Marine front, I have actually made some minor progress with my white helmet experiments. I tried out the wash+hand paint method, and I think I like it. I'll have photos and commentary up in the next few days. Last night was a rough night for putting the boy to bed, and by the time he actually fell asleep, I need the catharsis of smashing zombies in Dead Island.