I hardly use gas bubbles, net koopas, mechas and the coin game cloud (I don't even see why I should use that...)
But since I try to give every level a unique style in design and Idea, I tend to use sprites I hardly used before. For example there is a giant platform that will fly to the right forever or the skulls that swim on lava.
I mostly play other hacks, if I get in a situation like this.
But since being unmotivated is something that can't be controlled, try to take a break if nothing helps. Sometimes we need to go back to something with new thoughts, ideas or another mindset to be creative.
I think Dino Rhinos deserve more love. They have great potential for creative levels. I guess sprites like Dino rhinos or net koopas are rarely used, because in the original SMW they also are rarely used. Like only one level features Mecha-Koopas and Ninjis or the skull platform...