Hi Cyangmou, your insightful thoughts are very useful and the outline disappearing if I use the same colour as a background element was one of my thoughts I needed to try resolve. Part of the issue was anything that was a glow type effect such as the flame or other particle type effects.
Some other sprite enemy examples, with and without outlines. The spikes on the "Blackstar" would work quite well with outlines (not shown) and Particle Explosion example seems fine not to have outlines?
Appreciate your thoughts, I will continue thinking about this. I am more inclined to not have any background at all.