BIPs are either "standards track" (affects everyone, represents consensus), "informational" (ie basically just summarizing the authors viewpoints on things) or "process". 

My point is you can't have a credible standards track BIP until something has been implemented end to end. I don't think it's a good plan to design these things in isolation. You'll end up with bizarre user experiences because of technical decisions taken months earlier that are now hard to reverse. A working end to end implementation gives you the confidence to say, yes, this is how it should work, because here's the demo and you can see it works very well and the code is clean.

If your BIP is informational then no problems, but I don't think there's much point in informational BIPs to be honest - it's easier to just write an email or forum post summarizing your views on things. If you find it a useful framework to write your thoughts in that's OK, but don't expect implementors to follow what's written there just because it's a BIP. It carries no more weight than any other document would.