In terms of writing instruments, the greatest source of frustration for me comes from fountain pens. While I enjoy experimenting with different ink brands and colors and likewise find appealing the smooth glide of a good fountain pen on quality paper, I nonetheless find the overall fountain pen experience disagreeable due to one important factor: even with the finest nib size, I am forced to write much larger than what feels natural for me and as a consequence the quality of my penmanship goes to hell in a hand basket. Or in this case, an ink bottle. I have found that old steel-nib dip pens are more conducive for my style of handwriting than any fountain pen I’ve tried to date. Of course dip pens are about as impractical as one can get in terms of writing instruments.
Perhaps I just haven’t found the correct fountain pen yet. I recall reading somewhere that, as a rule, Japanese fountain pens have finer nibs that European or American products. Perhaps I should give an extra-fine point Japanese model a try (am open to suggestions!). Or perhaps I haven’t stuck with the fountain pen thing long enough to acquire the skill needed to write with the exactness I’ve grown accustomed to using .28 and .18 gel pens.
While I am intrigued by the world of fountain pens and bottled inks, I disappointedly remain on the sidelines, a mere spectator of new inks and “fountain pen friendly” paper, wishing that someone would develop a “tiny handwriting friendly fountain pen.”
Fine Points 
Check out the nib tipping chart at nibs.com. They measured the tips of the nibs, not the actual line width. It looks like Platinum/Nakaya and Sailor have the finest nibs available, at .10mm.
A ~$10 Pilot 78g F from Ebay might be worth a try, as well. That chart shows the nib as .35mm, but mine writes a line similar to that of a Micron .25mm if I write without too much pressure.
Also, nib grinding is always an option for pens that you already own. Research how to do it yourself and practice on some cheap Indian nibs (sold individually online) or have it done by a professional.
Thanks for the feedback. I’ve taken a look at the website you recommended and it seems that hope may indeed exist for fountain pen use.
Definitely try the Pilot Penmanship, and/or the Sailor or Platinum Desk Pens. They are quite fine and also inexpensive. Also, if you can get your hand on a NOS Sheaffer 330/440 in XF they have extremely fine nib as well. Try Peyton Street Pens NOS Sheaffers.
Excellent handwriting!
I´ve been using several Sailor Sapporos EF which are excellent and somewhat affordable pens. They may be what you are after.
The Sailor Professional Gear with the special Saibi Togi nib is my next step for a really thin line. It´s expensive though.
Esterbrook 2048 nib in an Esterbrook fountain pen is very fine and you can write really small with it. Also # 2556.