Some great thoughts here already.
The music (right now) is mostly Celtic traditionals. Not "pure" Celtic though as there are a few from North America as well as a few simpler classical tunes that I love.
Although it's based off my busking in flute and voice, I hear a mental accompaniment to most of the tunes I play, so I chose to do a mix of unaccompanied and accompanied. If I did a full album of unaccompanied melody I'd definitely want to mix it up and include both flute and voice and sometimes overdubs.
I have one album of music fully arranged (simply, though,) for flute, harp, occasional harpsichord or percussion - a few of those could be vocal lead rather than flute but it was planned out as instrumental.
It's all mostly ballad tempo to mid-tempo, none of them are superfast.
The thing is, most people are more impressed with my voice, it's more unique than the flute. But with my first album people liked the instrumentals more - I guess because there was a lot of variety anyway from them all being original compositions on different instrumentation.
(Plus which, it was all recorded at home without even a pre-amp, and I had to boost the volume quite a bit on the vocals, which caused a few artifacts that I didn't know how to take out. I've got a better recording system now and want to get professional mastering of the end result. (There's a few little studios in Toronto that have cheap rates and good results.))
I do want the album to be marketable enough that I can sell a few and make enough to keep making more albums later.
(Future album possibilities would be Christmas favorites (which pretty much has to be mostly vocal), original Goth rock songs (angsty and depressed, but that one will be done by a full band not just me and will be philosophical Goth, not vampires/werewolves/blood/death cliche Goth), more original New Agey stuff (which will be mostly instrumental)...)
What I do best is sing, play flute, and arrange/compose/orchestrate. I may need help with recording harp and possibly fiddle.
I create to please myself first (if I'm not pleased then there's no fun in it, eh?), but I'm sure that if selected and presented well it would be pleasing to others.
What I mean is, first there's creation, which is fun to me by itself. Then there's communication, which is trickier but shares the fun with others, and sometimes you have to go back to something you already created and change it a bit to be more shareable. :)