I don't mean to be a wet blanket, but the edge cases for the current markup syntax are already hard enough to implement correctly, to the point where different parts of Org mode are inconsistent. Intra-word markup isn't viable because there simply isn't any sane way to parse something like *hello world*/hrm/oh no*. The other issue is that this will degrade parsing performance because almost every character could precede the start of a markup section.
I recommend anyone suggesting solutions try to implement something that can parse the markup unambiguously with lots of nasty test cases. You will likely find that it is impossible to consistently tokenize markup, and that you have to hand write a whole bunch of heuristics, making Org syntax even harder to implement correctly. Any solution that suggests extending how =/*~+_ can be used gets a hard no from me. I could see teaching other exporters how to interpret \emph{hello}world, but trying for to have any sane behavior for something like why *hello*world oh no a wild askterisk* is not worth it. Best, Tom