I was thinking. How about we could nest the grid?!
For example if you press the "Add row" button and have highlighted a "cell", then you will nest that cell. If you don´t have highlighted a cell then it becomes a new row.
I´m currently playing around in the "panels.admin.grid.js".
So if you change this line:
var container = $( '<div />' ).addClass( 'grid-container' ).appendTo( '#panels-container' );
var container = $( '<div />' ).addClass( 'grid-container' ).appendTo( '#panels-container .grid .cell.cell-selected' );
Then you have nested a cell.
But then it takes a little more work to get it all to function thou. I´m no jquery ninja so i was thinking if you have any plans for this ?
Best regars /