Currently, object outlines can only have parameters of weight, colour, and dashing.
There should be more flexibility to defining the appearance of an outline. For example:
Something that can look like railroad tracks
The ability to stretch or repeat a vector
Consider the competition where an outline/curve can have a style defined by things like end-caps, corners, and in-fill.
I strongly support this request. As a professional user, two limits on outlines cause me constant work: an object can only have one, and it can only be a solid colour.
Stacked strokes. The current workaround is duplicating objects or text boxes. Every time the text content or the shape changes, I have to re-apply that by hand to every instance — inefficient and error-prone. It also damages artwork structure on import/export, since complex objects have to be broken apart. A stackable system like Illustrator's Appearance panel would solve both, and would make standard effects like text with multiple offset strokes routine.
Any fill type on an outline. Gradients above all, and patterns too. This isn't only an Illustrator thing — Inkscape has had gradient strokes for years. There is a workaround here, but it shouldn't be mistaken for a solution: Object > Convert Outline to Object (Ctrl+Shift+Q). The moment you convert, it stops being an outline — no line width to adjust, no dash pattern, no line caps, corners or arrowheads, no "scale with object", and editing the path means moving both sides by hand. You get the appearance, not the object, so anything you revise later has to be rebuilt.
One rule would cover both: each stroke in the stack accepts whatever a fill accepts.
One request: please make this complement the existing Text Background Fill rather than replace it. It stays highly efficient for non-encroaching outlines and we rely on it daily.
Finally, the demand here is badly fragmented. The fill-type half alone is split across at least four ideas — fountains on outlines (13 votes), Outline Fill Gradient for Strokes (10), Colour adjustments for outlines (8), Lines/Object Outline Gradients (2). That's 33 votes, nearly as many as this idea has on its own, and none of it is attached to the one that's actually being explored. Folding them in here would show what the real demand looks like.
And also to be able to add gradients, (fountain)fill a.s.o. would be really great
It would be nice to be able to have a thick line behind text so the text could be white - thus reversed out - and the line would stay with the text as it moves. InDesign has this.
And there should be option for double, triple lines in different configurations - same lines, thick and thin, two thick lines, one dotted and one continuous etc. with option to customize them.
And please be sure to add the ability to at least create dotted lines with circular dots. Currently you can either get only square points or dotted lines with round caps.