Hi,
Do you want to accomodate 3d content or 2d interface elements?
you will face not only a resolution problem, but also a ratio problem, the two are very distinct and solved with different technic.
3d content resolution: it doesn't matter.
3d content ratio : use a safe abstract area that fit between the two most extrem ratio you get get and fill the rest with background and props that are not important, but will occupy space on extrem ratios. I am not aware of any other technic without adjusting the content position itself.
2d content resolution: use a proper 2d framework such as NGUI or 2d toolkit, they have tools and tricks to switch textures. ELSE always design for retina, lower resolution will work ok. ALWAYS make your texture and sprite size multiple of 2 so that when they are downsized ( from retina to non retina), they don't get half pixels.
2d content ratio: use anchoring systems, so that content sticks to corners of your screens. GuiTexture and GuiText can do that, their x,y position ranges from 0,1 covering the whole screen area within that 1 unit position definition.
based on this, if you can refine your needs, I can go more into details.
bye,
Jean