Phonology Change?

I just had the darnedest idea for extending the Ceqli phonology.  I've been bothered for awhile that there's no zh sound (as in 'treaSure') in it, though there's a j and both a x and a c for the sounds in SHoe and CHew.  Symmetry demands something better.  I could set j to stand for zh, and then indicate the former j sound with dj. But if I did that, symmetry demands that I drop c and use tx to make that sound. Then I'd have an extra letter, c, that would be good for nothing.  So then I thought of, not adding accented letters, which are a nightmare, but using double letters. Since double letters simply can't occur in Ceqli as it is, double letters could be regarded as letters all by themselves, opening up lots of new phonemes — not that I'd use them all in Ceqli, but they'd be handy for ZH, and also for transcribing foreign words, especially names.

So let's say zz = the sound in treaSure
Then tt = TH as in THin
dd = THere
ff = Japanese bilabial F
vv = Spanish bilabial B

You see where I'm going.

Also, vowels could double, and
aa = cAt
ee = bUt
ii = thIs
oo = German ö
uu = German ü

Again, the only thing I'd regularly use would be zz.  I'd especially like to replace ja je ji jo ju with zza, etc.

Any reactions?