You might consider adding a couple more XML tags (or additional properties for existing tags) to be used in the Parameter section of the object definition.
Taken some cues from HTML/CSS, how about
float left
float right
clear
Using these tags/properties, you could arrange controls in horizontal rows and specify where to start the next row. It would be a start, in terms of allowing more controls over object layout.
Note: you'd have to move the parameter label above or underneath the control, for this to work (but to be honest, I think it would look neater here, anyway).
I used to be involved in the development of a video performance software, that had a plugin format based on Quartz Composer. At one time, they experimented with a method to allow plugin developers to customise the layout of their plugins, and even add their own background image. Adding the ability for Axoloti custom object developers to add their own logo or branded background image might encourage more established programmers (i.e. those who already have an established brand) to get involved.
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