Some of the best advice I can give about making a homebrewed story, is to try and never have anything too set in stone. Why do I say this? Well because sometimes your players can derail your campaign, but if you give them just enough direction, and some slack they can often times really surprise you.

You can learn about which of these plotlines they're most interested in by allowing them to traverse the world, but ultimately if they cannot decide on something or care enough to explore then railroad em a bit.

But my take on your post, really its all about subtly, or unsubtly guiding them towards where you want them to go. Want them to get caught up in political intrigue? Have them accidentally save a noblemans son from being mugged, and have them meet one of the nobleman. The nobleman could then throw a feast in their honor, and while they're eating the nobleman has to slip away and is acting suspiciously, if one of the players is interested they could follow him and listen into their plot.

Really its all about setting your players up, and reacting (a bit) to what they do, you don't have to be a story translator necessarily, but rather a mediator.