I've been contemplating this for a while, and I reckon that some of the earlier posters are right on the money.
You would need, physically need, to have all races playable. But think about what this could entail - can you really have an SM Assault Marine hacking some mob to tiny bits with cover fire from two Fire Warriors and a Necron Immortal? Of course not, it's batshit crazy, if you'll pardon my French.
What you'd need is, as has already been said, some sort of alliance system. You could have all the Chaos stuff - whether power-armoured dipshits, slavering daemons or multiple-headed primitives - lumped together, and have the Eldar and Space Emos - sorry, Dark Eldar as sort of the same thing, like if the Fall never happened, or just happened to a lesser extent (i.e. it just ripped the civilisation of the Eldar apart, rather than giving reality a red-hot poker up the anus and giving birth to a Chaos God). But again, that might be silly, and prone to inciting fanboy rage.
I like the points-to-spend idea, although that smacks a little of experience points to me and could easily turn the game into a big fat grind 'n' cheese toastie. Having said that, your idea of heroics earning points (like, if one plays as a Guardsman, taking an enemy turret with nothing but your squad, some bayonets and a massive death wish) which can be spent on boosting one's squad power with heavy weapons or Universal Special Rules (upgrades to the layman). Again using a Guardsman squad as an example, one could implement the Doctrines system, like upgrading everyone to Spartans - sorry, Stormtroopers, or allowing support fire calls from the Guard's one main advantage, the vast tank battery.
I can see that Space Marines would have to be nerfed a fair bit. So I came up with unit progression. What that entails is starting off with some sort of high-powered version of a Guardsman, then getting all the various upgrades to turn you from this super-Guardsman into a fully-fledged Battle Brother, complete with bolter, huge knife and powered armour. The same for CSMs, you start with a Traitor Guard and end up with someone like Kharn the Betrayer after someone reversed over his mum.
Implementing Bugs and Toasters (Tyranids and Necrons respectively) could be a bit trickier, but I think the answer lies in the squad system again. The Get Back Up rule could manifest as powerful health regeneration, and the Nids could start as Gaunts in massive broods that slowly get smaller as you go up the food chain, up to the point where you become a Hive Tyrant or Lictor or something equally alien and dribbly.
The DoW paint-your-own-gear would have to make an appearance as well. For example, Hive Fleet markings, Chapter colour schemes and Craftworld colours could all make an appearance. You'd probably end up with a welter of different eye-destroying players going at once, but with a limited palette for some races (Chaos God sacred colours, Dark Eldar's MCR T-shirts, et cetera) you could make that work as well.
In all, it could work, but only if executed properly and NOT, let us make clear, EVER EVER EVER by Richard Garriot or suchlike.