When I tried it just for a few days, I was surprised how easy it was to learn. It was much easier to learn than English. On the other hand so many people already know English vocabulary that just having regular spelling and simple grammar could work. Mark Twain already suggested it: "A Plan for the Improvement of English Spelling: For example, in Year 1 that useless letter 'c' would be dropped to be replased either b…
Agree, I would add, make vocals consistent, ‘a’,‘e’,‘i’,’o’,’u’ should only have one sound per each letter for every case, if we need more vocal sounds then use different characters.
This list was constructed by building short/long vowel pairs, and then tacking on the weirder vowels afterwards. As you'll notice from the inclusion of the last entry, there is a major dialect issue here. There are four sounds (/æ/, /ɒ/, /ɑ/, /ɔ/) that are usually distributed into three sounds in major dialects, and these arise from short a and short o in written orthography... but which words send their vowels to the different targets varies greatly depending on the dialect. If you pick it according to any one dialect, you're going to get some poor correlation in another dialect.
The other obvious issue is that adding new characters is actually difficult in the modern age. Of the sounds I've dropped in above, I can only type in ə and æ without resorting to copy-paste, and that's only because those are accessible with the Compose key. Telling people that you have to use 10 keys not present on their keyboards to write their language is going to be a hard sell. Again, note that English dropped þ as a letter primarily because it wasn't possible to print with printing presses imported from Germany.