Is there plans to insert functionality to allow publishers to control font sizes and type while configuring ArcGIS Online applications using the available templates?
There are some options depending on what you want to change. For something like the title or subtitle you can use html/css in the text box to change the font. for example you could do something like this for your title:
<span style="font-family: fantasy">Fantasy Title</span>
if you need more control, a few of the templates (including Basic Viewer and the new version of Elevation Profile) have a custom css parameter which allows you to override the style of different elements in the app.
Did you have any specific thing that you wanted to do?