... the one thing I don't understand is "why" and is there an "end user market" that you think would use it ... or is it just for you?
Perhaps I'm misunderstanding (I'm in the the "what's Facetwit all about" brigade)
If the plan is to market or offer access then perhaps the spec that the user might want or expect is a good place to start?
If you build a better mousetrap then you will have the world's longest running theatre show
"why"- because all the existing / previous sites I have used have had problems.
Bikejournal was slow, prone to locking out all the users first thing in the morning due to database mismanagement, had various small annoying bugs in the UI and the people running it said "give us money! it will get better!" It occassionally got a little better when they bought more server capacity, but as the number of users crept up it got bad again. I think that the Internet to it, in the US, to me, in the UK, coupled with some odd way they'd set it up made it more unreliable for me than most people
Mycyclinglogs didn't have as many features as bikejournal. For instance it didn't have a feature for tracking tyre use. The two major problems with it were that it had a week long outage and not all the data logged could be exported. It was faster than bikejournal however.
Strava had even less features than mycyclinglogs but allowed logging via GPS devices. Unfortunately, for the sort of rides I wanted to log ( short commutes ) it randomly added ascent and distance in a way that altered the stats gathered
I've used a couple of now defunct other logging sites in the past too, which had similar problems
So that's "why". There isn't anything IMHO that is suitable.
I'll build the site I am proposing as a prototype. Before I even build the prototype I will do a write up of what I intend to do and take feedback from that. Potential users will hopefully steer the design towards something good and usable.
Once the prototype is there I'll let some, limited, keen people use it. They will tell me it's crap. I will change it. The value to me is that I might potentially get a logging site I can actually use and also I will learn new stuff. I am not looking to create an end user market.
If it gets to the point that it is better than existing offerings then I will make it generally available. If it's any good and if it is reliable, people will use it. If it's shite then no problem
Unless of course it's shite and people still want to use it