If you want to "treat all {{ __...__, **...** and //...// }} as three different kinds of emphasis, without specifying the exact presentation," then why not support {{ '' this instead, or as well'' }} 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