One thing to be aware of with many commercial vegans cheeses is that these work much better for melting over pizzas or casseroles if mixed with some additional liquid (eg by soaking the grated cheese substitute in some soymilk or soy cream and applying both together).
If looking to make your own meltable vegan cheeses, use recipes using plenty of fat and a combination of agar and tapioca starch as a starting point for experimenting.
Be aware that the mouthfeel of melted cheeses is quite a complex matter: It is more or less defined by a set of textures achieved at different temperature points (and probably the delay in texture change!) throughout enjoying a portion (which will cool down during doing so. Body temperature also plays a role here...).
Be aware that cheese melted over casseroles has a functional role in cooking them - once it is melted (melting temperature plays a role here..) it acts as a partial vapor seal, and probably also gives off or binds moisture/salt/fat.
Be aware that the melting behaviour expected of conventional cheese has regional subtleties to it: Somebody used to american-style processed cheese in these roles could be put off by the liquid separation/curdling/firming/hard stringyness effects you get from using natural cheeses, while somebody expecting the latter behaviour would probably consider the more homogenous result from processed cheese as mealy/stodgy/gloopy.