.
If you want to "treat all
as three different kinds of emphasis, without specifying the exact presentation," then why not support Plugin insertion failed: Image plugin requires the name of an image and cannot be empty. since both tikiwiki and mediawiki use it.
According to Talk.BoldAndItalicsReasoning and a few other pages, __alternative title__ is ideal for titles including alternative titles as well. If you don't want to allow the user to mix emphasis, perhaps only the title should have a mixed emphasis? Such as, for instance, both underlining and boldfacing ? Then you can have _just underline_ as well. Or do it with three and two respectively, if you don't like single characters to mean anything.
See if MakeTheMachineWorkHarder expresses your views on this, if not, make clear how you feel about it.
--Anon