Creating your own design can be really rewarding, and deserves thorough planning and attention to detail.  This always takes a bit of time, but at the end, you know that you've got something truly unique.

Creating your own garments, motifs and patterns is good fun. Most things can be knitted, although it probably doesn't need pointing out that the simpler designs are easier to knit! 

Garments - this is best done (in my experience) by starting with a pattern and altering, either by adding or removing bits, or putting together a medley of patterns. You need to be cautious though, when combining patterns that the yarns and tensions in the original patterns were the same, or at least that you've planned to allow for any differences. But don't be put off, it's all possible with a bit of forethought and planning. 

Motifs - it's usually quite easy to take an idea and see if and how it would work in knit, all you need is the idea and some graph paper. 

Patterns - Fair Isle style patterns are having a huge surge of popularity in the shops at the moment. True Fair Isle has just two colours per row (although as many as you like in the whole project), and more than this get very complicated to knit. 

Lace - I've never created my own lace patterns, and I'm only just starting to learn how to knit lace pattern. But bring me a pattern, I'll give it a go!