There are at least two very deep rabbit holes with fountain pens: inks, and vintage pens. Writing with an ink the color of Crimson Glory Vine, for example, is

There are at least two very deep rabbit holes with fountain pens: inks, and vintage pens. Writing with an ink the color of Crimson Glory Vine, for example, is a meditation all on its own (Pilot Iroshizuku Yama-Budo). The vintage pens rabbit hole tends to force you toward inks that are "safe" for vintage pens (Waterman Serenity Blue, for example). But either way, what you write with them, and what system you use is constrained only by imagination.