Drop OpenGL and port it to D3D9.

Seriously.

OpenGL on integrated graphics is getting better for sure, but it's still not in the same league as D3D for robustness of driver quality and all-round general predictable behaviour. D3D will give you (and - importantly - your players) a much more stable, solid experience and with less driver weirdness to have to deal with on integrateds, you'll be able to access features that are just not available with their OpenGL drivers (such as high-level shaders), and you'll be able to target machines with OEM drivers that don't include OpenGL support. D3D9 in particular will enable you to hit even older Intel GMA class graphics with a reasonably full feature set.

On the other hand, and being a little more serious, I'd suggest that you're spreading your target hardware too wide. You're talking several generations of hardware and at least an order of magnitude difference in performance, and you want to get the best possible out of it all. Even with the best will in the world that's just not going to happen; nobody does that, not even major game studios with huge budgets. So scale down your ambitions and pick a more realistic and achievable goal instead.

No, DirectX is Windows only. That said, it sounds like you may be a beginner of sorts. Perhaps you should aim for a smaller scale project to get an understanding of the graphics API before you target something as large as an MMORPG?

OK, but I still think you're taking on too much here. Let's recap.

You want this to be portable, and presumably as well as Windows and Mac you're also looking at Linux.

You want it to take full advantage of modern/high-end graphics cards.

You also want it to run as well as possible on low ends and integrateds (which are also going to include those integrateds with OEM drivers I mentioned).

You're just not going to be able to do this, full stop. It's neither a realistic nor an achievable goal. Yes, it's nice to be all inclusive, but one person dosn't have time or resources to do it.

I have seen good OpenGL performance and compatibility in the newest Intel integrated GPUs, like HD Graphics 3000 and co. These will eg. be able to do deferred rendering without problems and with reasonable performance. Anything older than that (for example GMA 3000/4000, not HD) will be problematic and works much better with D3D. Note I'm only talking of Windows drivers here, have not tested Intel cards on Linux or Mac.

To scale performance consists of many things, in addition to texture resolution you can eg.

- reduce complexity of shading equations

- leave out "decoration" objects on low-end systems, as well as particle effects

- have object LODs with fewer triangles

- disable dynamic shadows, or render them with lower resolution

- disable or scale down post processing effects

Basically, everything you usually see in games' graphics settings menus.