In general, my goal with this little project is to design a mind virus that captures the benefits religion confers upon people, while doing away with its most grievous harms. If you've heard Patton Oswalt's bit on Sky Cake, then the analogous goal is to build a Sky Cake that is genuinely better in every way. The challenge of building a better religion is that it must preserve the good that religion does for people and in the world, while at the same time staying consistent (or at least out of conflict) with non-religious accounts of ethics, astronomy, etc. In other words, the consequences of this reverse-engineered religion must come out good on a humanistic analysis, satisfying the moralities of atheists and sparing their intellects wherever possible, while at the same time satisfying the emotions and faiths of the believers.
Perhaps the greatest perceived benefit of religion is that it is able to help some people come to terms with their mortality. The best possible religion must retain this benefit; this may also be accomplished with reason alone, and atheists are living proof of this. However, the mechanism by which religion accomplishes the same feat is an emotional one, not a rational one; whatever the cause, reason just doesn't do it for believers. So the best of all possible religions must deliver an emotionally satisfying account of mortality which appeals to a naive sense of fairness (so that naive people will buy it), while at the same time actually conferring the rather unique tranquility that comes with knowing that one shall die and truly accepting that fact with a calm heart. Remember that I am not forsaking sophisticated and non-religious morality - appealing to the naive as well as the sophisticated is a challenge and a half, and I'm trying to take it all in one bite here.
A related benefit is the problem of evil: atheists deal with the problem of evil in the only way they can, i.e. with reason. Evil exists, and with no observable intelligence behind the Universe at large, we ourselves must be the ones to work against it at every step of the way with no outside help, unfair as that is. Again, while this is intellectually sound and, in my experience, tends to bring a modicum of steely resolve to one's life, it lacks the emotionally satisfying purposive account of evil's origin which believers seem to crave. If this is to be a better religion as a religion, then the emotional impulses that keep so many in the firm grip of faith simply must be satisfied. Remember, I'm playing for keeps here: religion's hooks are many, and dirty, and they strike deep. To succeed in this task, I will need to be even better at this game.
Another benefit of religion is social cohesion. While equanimity in the face of inevitable* death and evil is of great personal benefit, social cohesion is an even greater one, though it tends to go relatively unnoticed. The fact of the matter is that human beings always find a way to foster a sense of community; religion codifies this in tradition and shared beliefs, which gives it more long-term and far-reaching effects than memes such as, for example, Local Sports Team Rocks, or Tuesday Night Parties Are The Shit. Any religion may be believed by anyone, anywhere, at any time; and so the best of all possible religions must foster a greater sense of social cohesion than even this transcendent bond between believers. The best way I can think of to accomplish this is by strict moral injunctions to embrace outsiders - or in other words, to foster total social cohesion for its own sake, and not as a fringe benefit to members only. Exclusion must be demonized, and inclusion exalted, for this religion to stand out among its peers as a force for helping people get along even when they're not all getting their way.
These are the purposes towards which this experiment shall be bent: encouraging acceptance of mortality, providing an emotionally satisfying explanation of evil, and the fostering of universal social cohesion. These purposes shall be pursued as ends, and the means to them I shall describe below as the mechanics of this religion.
Concision. Religious texts in general tend to be long, boring, tortured, and self-contradictory. This thing I'm doing here, yeah, it's not gonna go like that. People simply have better things to do with their lives than hole up in a monastery and copy scripture, and a modern religion needs to take this into account. There will be a holy text, and it will be full of mythic storytelling and instructive parables and so on and so forth. But these principles, unlike those that developed orgainically from naturally occurring religions, have the advantage of a single author with an eye to consistency: me! Also, I can make sure that I'm not promoting outdated cultural mores, as so many religions do. My intention with the short holy text is to cut out the riff-raff of boring nonsense and backwardness that plague the literary meanderings of most (if not all, I don't know for sure) religions.
Clear Humanistic Moral Priorities. Any moral theory worth its salt will be consistent, coherent, and well-reasoned. Remember, however, that though this reverse-engineered religion is designed by reasoning processes, it is not designed to appeal to reason. Instead, the morals will be conveyed along emotional lines; it just so happens that all these emotionally-phrased moral codes will have solid rationale available. In other words, the reasoned path to this new religion's morality shall be clear and available, but it is a long one; so the shortcut of faith, which so many claim to use for their morals anyway, shall be made available to those who lack the time, intellect, or inclination to engage in such philosophically oriented pursuits. Hell, I'll even include some good solid rationale, and make this the more esoteric part of the holy text, bottom-lining it for the lay reader in clear and certain terms in the "mainstream" parts.
Peek-a-Boo God, or "God of the Gaps." Whatever the creative force behind reality as we know it, it has to hide in gaps. These gaps are continuously narrowing, so I'm thinking I'll have to posit a parallel universe which cannot be accessed by mere mortals, and can only be conceived of by "secret knowledge." The point is that this is supposed to be unfalsifiable, but not in conflict with scientific knowledge (whether currently available or forthcoming). This is still a tough row to hoe, as the gaps of science are a moving target - or rather, an ever-shrinking target, but shrinking in unpredictable ways. I basically have to predict where science shall tread, and take care to deliberately avoid crossing paths with it for as long as possible. I think that avoiding conflict with science is going to be the hardest part. This is why, in the spirit of the above (on morals), I shall also make explicit the below:
Deference to Science. In point of fact, the best way to learn about the world is to look at it. However, how we look at the world counts, and so our investigations will perforce be constrained by the ways we are able to look. If we humans, or even just one of us, received allegedly divine communications which were reliably borne out by reality - in other words, if miracles were promised and then unambiguously delivered - then this would be a perfectly acceptable means of looking at the world. However, to the best of our ability to determine whether or not this is actually the case, it appears not to be the way the world works. And so we must ply the world for its secrets with silky smooth science. It is important to understand this point, whatever we are investigating, and be sure to investigate the world in ways which involve testable, tentative ideas. This means that our reverse-engineered religion will have to be genuinely adaptable, while maintaining the appearance of claiming to have access to the One Truth of the Universe. Peek-a-Boo God helps walk this razor's edge of competing values - believing we have the One Truth of the Universe, and being willing to defer to science as needed - but there is no way to guarantee a permanent solution to this problem, as some ideas are simply not compatible with reality once we discover that bit of reality. And, science being the progressive up-and-comer that it is, we still have quite a bit to discover and we can't say for sure what metaphysical propositions will or won't (or even "can or can't") be ruled out by scientific investigation.
This, to me, is the most important point: I want to do my damnedest to avoid the asinine chicanery that is Creationism, Geocentrism, Flat Earthism, and so on and so forth. However, as a religious advocate in this highly specific case, I also want to overstep the bounds of legitimate science. My goal in accomplishing this tightrope walk, as mentioned above, must take the form of abject deference to science in all empirical matters, casting my supernatural claims as far from the realm of investigable phenomena as possible while still remaining intelligible to the general readership. In other words, atheists will remain unconvinced (and justly so, for we're right), but all the world should be able to see that this is a damn sight better than any extant religion on any sensible analysis.
* - Here I mean only that some death and some evil is inevitable; these problems can't be "solved forever" and we will be better off if we can accept what we truly cannot change while still changing all that we can. Tough line to walk, I know, I'm just saying that we should give it our best shot and be happy with that (unless we think we can do better, in which case that wasn't really our best shot, now was it?). In other words, we should try to be happy with as much good as it is in our power to do - no more, and no less.