I went micro first, and then ultra wide, and I found a couple of things.
First I bought the 105mm micro. At the time I owned it and the 18-200mm. It was a good combo, but I soon found myself using the 105mm for anything that 'mattered' because it was a more useful lens - sharper, faster. And I found myself using the 18-200mm at either 18 or 200mm.
Then I got the Tokina 11-16mm and I totally agree with other posters in that shooting 11-16mm is totally different than shooting 18mm - beyond the numbers - the ultra wide is far more, for lack of a better word, trippy. It's also a great quality lens.
So now to your question. While I'm not a 'pro level' macro shooter, nor am I a 'pro level' landscape and wide angle shooter, I've gone a ways down both roads.
For wide angle stuff, if you have the lens and a good tripod and the oomph to get out shooting, you're pretty much set. You could get a pano rail for your tripod, filters, and that's about it (I think).
For macro, things can potentially get a lot more expensive. Macro rails for your tripod, a tripod that can sit at ground level, lighting rigs that look like weird alien machines, software for focus stacking, studio setups... There's a lot of gear growth in macro. Sure, you don't need all of it, but the potential is there.
I've had a blast with macro. It is a great way to find color if your neck of the woods doesn't have much (bugs, fungus, all sorts of crazy stuff when you're at 1:1). Wide angle in a city is proving to be equally fun in a different way for me, as making buildings do weird things to create atmosphere is very cool.