As for the more general "what do I do next" advice you're asking for? Well... there aren't clear cut answers for that, unlike the above two essays I just wrote. I know it sounds trite, but all I can really say is, "It's up to you! What do you want to do?"
Survival is something that you can get a formula for. Making a sweet fortress to be proud of, however, is not a mathematical formula like survival, where there are efficient ways of doing it and inefficient, right and wrong if you will. Making a sweet fortress is a matter of ART, and taste. You can follow in the footsteps of other fortresses, and try to imitate methods, reproduce results, but there are so many things to do, and so many ways to do it, that I can't tell you what to do next. I can only offer some guidelines.
Guideline: A military is a good idea eventually. If you want to pander to the nobles, it'll even eventually become a requirement. But there are other ways of defending yourself. Siegers can't swim, for instance, so a 100% effective way of defending from siege is via a moat/drawbridge. There are other methods. Now, perfect defenses like those can be boring, and may fail to appeal to your sense of aesthetics, so you may want a military.
A trap system will likely suffice for defense against most (though not all) river, chasm, and magma monsters. If your dwarves are industrious enough, they can churn out enough traps to cover every square of your entire fortress that isn't needed for other things. That's WAY more than enough.
You'll need a darn good military if you ever want to cross the magma though.
Guideline: Eventually, you're going to want more workshops, particularly if you want to start the dwarven economy. Glassmaking is a very VERY good idea, because green glass comes in endless supply when you get a magma furnace, and you can make almost anything out of glass. Furthermore, glass is often asked for by fey dwarves, so it's a good idea to have some in stock. Clothes making will eventually be needed, as clothes wear out, and when your dwarves are naked, they're unhappy. And yes, their clothes will in fact literally disintegrate off their backs after a while. Alchemy isn't really important at the moment. The farmer's workshop can help with food management by letting you stop planting plump helmets which your dwarves will eat raw, and letting you plant crops that need to be cooked, so you can make them happier by forcing them to eat cooked meals. And yes, sometimes, they need to be forced.
Guideline: Getting to the magma is important eventually, because you REALLY want magma smelters and furnaces, otherwise you WILL run out of wood. I don't care how heavily forested the map -was-, you'll clear cut it in no time. This requires steel, of course, which can be... complicated... to make. Check out the wiki for more details.
It'll also require decent defenses, either in the form of a military... which to be honest, would benefit from the magma furnace in the first place in order to get them steel weapons and armor, so I personally prefer a military after I get magma furnaces up... or in the form of massive amounts of traps, which are easy enough to make and install.
Guideline: You need to think about nobles that will be coming your way. Some of them, you may desire to drown, and as such, you'll need to build execution chambers. Or maybe you want a challenge, and decide to keep them alive. Either way, there are SOME nobles that are just plain useful any way you look at it, like the broker and manager, and you'll have to work towards accommodating them. Dealing with noble needs and desires will take up a fair bit of your time and effort and planning, and may influence other goals.
The above things are just guidelines and things to think about. As you keep playing and get deeper into the game, you'll start thinking of new plans and strategies and work out aesthetic and practical goals you never even considered before. For instance, have you considered setting up a TEMPORARY fortress near the entrance, digging back to the magma, and moving your entire fort back there? This will improve the efficiency of your magma smelters 10fold, as the smiths will no longer have to walk the long miles back to bed, to dinner, to drink, etc. Of course, doing so has its own design considerations to think about and implement, defense issues to work out, logistics, etc...