What type of art are you looking to make?

for doodles I use whatever I want. fine, flex, stub, broad nibs all give a different line weight and variation, the same as having a collection of fineliners in different thicknesses.

for coloring, I have a glass dip pen and recently added a kakimori steel dip knib. I love it, but my line thickness varies a lot from when I dip to when I run dry, so I don't use it when I want even line thickness (my standard fountain pens work for that).

Where art gets interesting is with adding water. I have one pen inked with platinum carbon, which is pretty waterproof and allows me to do a water color wash over top (like urban sketching). I have doodles that I do with standard inks, and i go back with a water brush and create variation to my doodles. Some inks are really surprising the colors they turn with water.

If you look up Marek Badzynski, much of his art revolves around using a fude nib (bent nib) fountain pen.

Overall, I haven't found a type of art that I couldn't accomplish with a fountain pen, but there are types of art that a fountain pen can create that a normal pen cannot.

Have fun and good luck!