Currently as i understand, the engine allows you to have one and only textbox image per game.

First idea, 

How about having the possibility to have one text box per character. Basically declaring the text box image assigned to a character, in Setup.yaml like in this exemple :

----------

characters:
  deuzi:
    displayName: Deuzilene
    speechColour: "#ca90cf"   

     textboxes:    

         textBox_01:  assets/characters/textbox/deuzeline_TB_01.png

              position : 300 600  <!--  to change the position of the text box image and its text content  -->

              namebox: no   <!--  No/Yes - For the name to be not displayed -->

        textBox_02:  assets/characters/textbox/deuzeline_TB_02.png

              position : 300 600  <!--  to change the position of the text box image and its text content  -->

              namebox: no   <!--  No/Yes - For the name to be not displayed -->

    looks:
      normal: assets/characters/Char3Normal.png
      happy: assets/characters/Char3Happy.png

----------

For this first idea,  the vision behind, is to have a text box for classic Visual Novel dialog, and another one to be displayed around the character head to have like a sort of comic strip speech bubble to show like inner thoughts by example.

Second one,

About the lines with no character ( text : My text is here. ) The text box is the main text box. It would be nice to have a different one for this context or better, having text box image displayed on some condition using a variable, and cherry on the cake, having the possibility to declare a position for this one.

 The engine is great like it is. That's just ideas, i come up with when making this new game with RenJS.