The orthography of Modern Greek is to a great extent historical and, therefore, complicated. There are multiple spelling variations for [i], [e] and [o] sounds, and awkward digraphs to represent [b], [ɡ] and [d] sounds. Have there been any proposals for a spelling reform for Modern Greek? If yes, what kind of a new orthography do they suggest?
Yes, there have been suggestions of reducing υ, η, ει, οι etc to ι since that is the way they are pronounced today. Also ω to ο and αι to ε. Nevertheless, orthography in Greek is kinda "holy" and such suggestions are fiercely opposed. One of the arguments is the readability of ancient texts in the school (kids should recognize the words easier). For the consonants like β, δ, ζ there is not much to do as it is not a modern sound change anyway. The discussion has always been around the vowels.
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.