I think Egis need to break way from their very basic function. As they are now, Ifrit and Garuda are basically an extra auto-attack and that's about it. Garuda has the benefit of Contagion, but otherwise nothing stands out. I think Egis need to have an actual function and some tactical use.
As for Erosion, they could always make the special effects of Erosion and Contagion exclusive so that one over-writes the other. They won't stack. And there is nothing OP about the idea of Erosion, it could have a very low base potency and be relatively useless unless you stack some DoTs first. That would be its purpose and use.