I'd also say trader works well on the remodeller cards, such as Upgrade, Remake, Develop.
I've found that the two problems with those cards is that sometime you don't have anything good at a $ level to move into, and that you often end up with hands of $2 or less. Trader helps both those situations.
One combo (which may be a nombo) I think might work on mid-strength kingdoms is Develop-Trader. I find Develops real weak point is when you Develop a $3 card, as you often end up having to stack an Estate. With Trader in there you could stack a Trader and a Silver, and Trade the Silver next turn, or if Develop and Trader don't collide, a Trader and Estate, and Trade the Estate next turn. Not sure how smooth this would be, but I reckon it could work.
I don't think its a complete strategy on its own, but if you start mixing in other decent cards to Develop into, it could start to click.