I'm not sure that it makes sense to have the transformations symmetrical around the base tile.

The reason is because it's easy to just extend the frame in just one direction, but a pain to have to have to make identical handles in all directions. It's for designs where the user wants something to fade in all directions, for example.

... tiling of gradients? D:

As for the interface for actually adjusting the transforms (as well as opacity and such). Is it necessary to do it with "tiles" shown on screen, or would it suffice to allow transforming actual tiled clones?

Hm... actually, live preview of just the tiles could work? In that case, the handle will have to be the tiles themselves. (option 1 of my three suggestions).

You have a point about jitters. I don't think small amount of jitters would affect things much, but it's true that some users may want to make larger amounts of jitters.

In general, I think the system is very interesting, but I would be interested in making it a bit "leaner". After all, someone has to implement it, and (more importantly), a lot of someone's have to learn it. So the more we can leverage existing tools/infrastructure/concepts, the better.

True. Adding handles and having the transformation fan out in all directions are also features not allowed by the current interface, but that doesn't mean I can't leave them in there as features that could be implemented eventually.

I do plan to decompose this into smaller tasks once everyone agrees on the general interface. For example, the bare-bones version of this feature would only feature normal clones (no fuse mode) with wallpaper tiling (no radial, no lines). Instead of on-canvas dynamics control, users just click on defined target tiles to pop-up a modified version of the current interface, and input values manually. etc.

To be honest, I'm not even completely sure how Inkscape's current interface works. That's why I started a re-design. Once you've looked into the current design, it'd be helpful if you could tell us what's actually going on...