There's no obvious plugin to add leading edges to transitions that I can think of.
However you can simulate a range of transitions by putting effects on the layer. Here's the steps:
Choose your effect (e.g. Sparkles) - drag it to the layer
Configure timeline on opacity: play once, play backwards, speed = transition time, in-outs and easing to get the transition you want.
Configure the other effect params with timeline transitions to your liking
Now when you change clips on that layer the the effect will play over the actual transition (or even without a transition). You can get a sort-of leading edge for some transitions this way (try Blow or Grid). Not perfect by any means though. Could do with a ping-pong and stop to make it more transitional.
This brings up the idea of transition configuration. Oftentimes I have wanted to change the parameters for a transition (e.g. Noisy or LoRez) in the same way I can configure the corresponding effect. Having some sort of config for transitions could open up ways of creating George Lucas style wipes with edge effects like colour and blur.
