Unclosed tags will not lead to any issues with how STM currently works, and if I were to add this tag stacking change I would also not require tag closing! As it stands, when text parses, it just goes forwards, not backwards.
(There's some benefits to the current system by the way... color tags of a different type can stack (color, gradient, texture) but now that I've got the combo color tag (<c=myColor,MyGradient,myTexture) and customizable color-mixing rules for color data, adding an alternate tag parsing system could be interesting...)