Looking closer at the decals on the Lego "Ultimate Ultron" it looks like it uses the same template as Ultron Prime (basically the armor shapes) only the main difference is that it almost seems like he's covered in molten metal based on the wavy pattern (complete with the edges seemingly dripping down unevenly):
I know that folks have mentioned the idea of Ultron using molten metal to coat his external frame (what with the trailer and all) so I thought I'd show this as it could very well be an indication of how Ultron will end up (literally) covering himself with vibranium. I can easily see this form turning out almost the same way as Ultron Prime only with a smooth/wavy external frame and less visible panel lines. Would be very cool to see how it turns out and for HT to make it.