Currently, Solarus has the option of "Immediate", "Scrolling", "and "Black Fade" for transitioning between maps. In A Link to the Past there were several more transitions like "Pixellated Fading", "Spotlight Zoom", and "Choose Color Fading" used between certain maps.
Pixel fading was used in A Link to the Past to move into Zora's River, Skull Woods/Lost Woods, and under the bridge.
A fade to white was used to move between the light and dark worlds.
A fade to blue was used to move between whirlpool locations.
The Spotlight Zoom was used to move between entering and exiting houses, caves, dungeons, and secret passages.
Will these transitions be implemented in future builds?
Maybe this will be feasible from the Lua part, when we have pixel-access in future versions of Solarus. That would be the best for customization.
I thought most of these transitions can be made with images? I think pixel-access would be good because it will be obvious that pixellated Fading is fake with images. Also, being able to manipulate pixels has many more uses. You probably could simulate this with 1x1 image blocks and random variables.