The earliest such proposal was Ioannis Vilaras' in 1814, proposing eta (not iota) for all /i/ sounds, and omicron for /o/: https://en.wikipedia.org/wiki/Ioannis_Vilaras
The most successful such proposal was in the Soviet Union in the 30s, when all Greek (including Demotic, Pontic, and Mariupolitan) was written phonetically in the Greek alphabet (with upsilon for /u/ and universal final sigma)—the latter two variants with new digraphs like ςς for /ʃ/. Those digraphs have been revived in the Pontic Wikipedia's orthography.
The digraphs for voiced/(optionally) prenasalised stops are arguably awkward, and transcription of dialect can dodge them; I'm not aware of orthographic reform proposals taking issue with them, though.