D20 magazin would be really cool, but I agree it must be a physical one, not a e-zine (I personally wouldn't pay for a e-zine and would be rather unlikely to read it).
Things that I want it is mostly articles on various D20 related topics (be it products, industry, D20 gaming thoughts etc), also rants like the one monte cook does every now and then on his site is mega fun reading and very interesting (especially when concerning D20 and the RPG industry heh).
One more super important thing as already mention, is professional look, editing, and so on. Without that it wouldn't be what I pay for, and I am sure others feel the same.
Another thing I like is relative small previews of upcoming products, maybe they will be obsolute, but gives a good idea of what to buy and what not to (that said don't devote too much space to such).
Theme issues are an excellent idea, sure sometimes one wouldn't like an issue but that happens anyway.
as for stuff about "famous" rpg people's escapades, umm no thanks, if such things should be included let them describe it themselves IMHO.
Cover other game systems idea described below is totally uninteresting IMHO, sure I LOVE Marvel Super Heroes, but if I am paying for a D20 mag I want to read D20 related material.
I wouldn't mind a single story or game play example in each issue, but it should be kept to a single or two, anything more (especially if they are long) takes up too much space.
Finally stuff that expand on exiting stuff is fun to read, various crazy ideas, not something to stuff the mag compeltely with, but a few thigns are good, also ERRATAS would be nice to see in there.
well just my opinion on a D20 mag.