The main obstacle is that here are far more sounds than letters, and some sounds have no letters at all. This is particularly true in the vowels, where there are about 18 vowel sounds, and only 5 letters to write them with. But it’s true for consonants, too. That means that we are going to have to invent some completely new spellings, which is in direct opposition to the goal that regularly spelled words should be unchanged. But the first 2 principles are more important, so let’s bite the bullet.
There are 2 sounds that share the spelling ‘th’. There’s ‘th’ as in ‘thing’, ‘thought’, ‘thigh’ and ‘th’ as in ‘the’, ‘there’, ‘thy.’ Indeed, the last of these, ‘thigh’ and ‘thy,’ differ in no other way. So we need to make up a completely new spelling. Ick. Since we need voiced and unvoiced ‘th,’ we should use the voiced and unvoiced pair ‘t’, ‘d,’ which gives us ‘thing’, ‘thought’, ‘thigh,’ ‘dhe,’ ‘dhere,’ ‘dhy.’ Already, after only a single thought, this is starting to look very ugly. But let’s soldier on.