>>78
You don't need to aim for a PHONETIC spelling... but a PHONEMIC one. Like, the K in Kill and in sKill sounds pretty different, but it's the same phoneme (sound building block), so it's OK to use the same letter for both... most dialectal variation is just about this, and, if you consider a lot of different dialects while making your spelling, it'll probably be OK.
About ð and þ: just using "dh" and "th" would be the best bet... if English didn't already used "h" for a sound and didn't allowed words like "courthouse". So, these letters can be really useful...
I would suggest "make slow, make small, make progressive". Changing first things that are already against English orthography rules (like island>iland, piranha>piranya, machine>makine, etc.). After this, tweaking the rules a bit until they're acceptable.
One thing I never understood is, why English tries to write loanwords as they were in their native language? Does writing "filozofy" instead of "philosophy" wouldn't be already a progress?