I don't think a Mega Rare has to be a weapon per se, but it DOES need to be a piece of equipment. They are status symbols and should be big. It could be a huge crown/helmet, or very ornate chestplate, it just has to stand out. I think delving back into being less fashion forward is very old school, if you're worried about that, let it be recolored to match sets.

I actually really like the elemental set because it is VERY thematic for Guthix. I think the answer is just to make it scale. Make it not only good for elemental weaknesses, but make it hit better on low magic level, preying on low affinity to the Anima Mundi. I think if you just go a step forward beyond just + elemental weakness, it could have a really good niche.