Historically it seems the difficulty from crafting was from the RNG of skill usage; ie did the skill fail because it wasn't a 100% success rate skill. To me, that is a horrible system. If you use a skill it should succeed; someone should never have to wonder if their craft will fail because of a skill success rate.
Difficulty should be expanded with RNG in different areas such as quality RNG procs (Excellent, Good, Normal, Poor).
First it should expand quality procs to have a 5th one so there is a balance of good and bad procs.
Second there should be an equivalent spectrum for progress.
Third there should be elemental procs.
Then we need skills that will take advantage of each proc in different ways. Either harness the power of an excellent to brute force a huge chunk of progress/quality or exchange it to alter the procs on a different area such as exchanging an Excellent Quality proc for a Good Progress proc or exchange an Excellent Progress proc to prevent the next Poor Quality proc. Give us the skills to make trade offs between Quality and Progress.
Elemental procs would be a similar to the Whistle system or IQ stacks. As you work a craft you're imbuing it with aether and as aether reaches a certain amount of saturation you can spend it on altering a proc or enabling a proc based skill when it wouldn't normally be usable or let the aether sit there for a minor persistent buff on certain skills.
At its heart crafting becomes only slightly harder or more complex; if you want to ignore it you can still craft something just like macro crafters do now but you'll be unlikely to HQ something (maybe 20% quality upon completion) but if you try to master how the procs play against each other then you'll have an easier time making HQ. The difficulty comes in learning to play the trade offs than a rotation.
The difficulty in crafts could be changed by altering how often a proc might appear for that craft and not just increasing the numbers at the end. Maybe for Expert crafts the excellents proc half as much but poors proc twice as much. The difficulty will be in managing your procs to best effect, not just getting bigger numbers.
For easier crafts, maybe elemental procs don't appear so you have less to manage or maybe excellents appear more often so its more forgiving of mistakes.