The more variety you have, the more happiness you gain, so I go for many different things.

If you really want to be in for the long haul, focus on producing the items that may be requested by the omnitemple. Then those things it isn't currently requesting can be sold as luxury goods.

The way I like to play is that I customize the research cost for everything by making them all x10 and then I focus on trying to complete all the research.

Late to the game, but I focus on the highest coin/happiness/industry/research items. So for example, instead of just bread, go for sandwich, fish stew, x10 research items instead of the x1 books. Then if happiness start to sag, I'll make a smaller production line for a smaller item just for variety.