It’d be fantastic to see an extension for more effects or customizable effects! If you want an example to reference, @jwunderl made a small extension to add an effect that is like the rainbow “trail”, but it takes on the colors of the sprite it’s added to: https://github.com/jwunderl/arcade-particle-effects/blob/master/extra-effects.ts
Our in-game effects are defined in this file: https://github.com/microsoft/pxt-common-packages/blob/master/libs/game/particleeffects.ts#L257