Strobes can give you a different effect. You can control the light in a more precise way. You can use them as a fill, second light, overpower the sun, just to illuminate some details you want, softbox, beauty dish, snoots... If you go for strobes, don't forget you'll need the light stands, adapters, triggers, etc...
And you don't need them to be canon strobes. If you buy a cheap trigger system, your camera wont be able to communicate with the strobe. If that's the case, an used Nikon one is a better option, as they are better build.
Still, i would recommend to go mid level. Yongnuo has some great triggers that will allow ttl and control the flash from your camera (if your camera has that option). That way, if you position the flash in a difficult place, you can zoom, change the power, change the mode, etc, from camera.
But a strobe it's not necessary the best option. If you look for a more natural feeling, the flash can take that away.
I'm not a big fan of the 70-200 f4. I would go to the 85 or tamron 90macro.
I would buy a macro only if doing close up and have a great make up artist. For casual portraits, it may be a little too much.
The 50mm in your camera is almost an 85. Don't you feel the need to open the image with a wide angle lens?
http://pedro-antunes.com/albums/black-princess/
Some times, the wide angle give's you a different view on the space and helps you frame the subject on the space.
Well... In the end you have to see what YOU need. If what you have is working for you, why would you need to buy something else?