Do you want the water to fall from the waterfall bucket directly into the pond, or will you try to make a multi-level spillway with your rocks and have the waterfall bucket as a reservoir at the top? A single spill from the bucket will be easiest from a mechanical standpoint, and you could surround the bucket with rocks to make it look prettier... with multi-level, you need to be careful to angle your stacked stones slightly so that the water all flows down into the pond... I've never actually made a waterfall, so other people might have more practical advice.
As for the placement of your waterfall... I think the automatic thing would be to put it back toward your corner (like where the turtle spitter used to be). However, you could consider placing it at or near one of the ends of the pond also, and then you can use the depth of the corner for your plantings. I could see a cascade entering about where that streak of light is falling in your photo of the empty pond, just to the right of that large indentation coming down from the rim of the liner. Again, this depends on how complex your waterfall will be and how much depth it will take up. Will you go all the way back to the wall? Maybe you could even use the low wall to help support the upper level of your waterfall design...
It was a beautiful little pond even before you took it apart, and I think a waterfall will be a wonderful addition! You'll get ideas as you go along, too... I think my biggest practical suggestion is, don't fit all the flagstones back into place around the edge until you have the waterfall together and running and like its placement. You don't want to rearrange those big slabs more than once!
