It is not common that alphabets are simply created by one person. Most scripts are taken from other cultures and adapted and evolved. Only the oldest scripts evolved out of ways to record something, but then become complex and eventually come to represent the languages of the people who created the writing. Most cultures find it easier to take and then adapt a script to their language(s) which then results in new writing systems.
For those that are created in whole by a single person, this is called “Stimulus diffusion”, where an idea inspires the creation of something similar, but that end result is not descended or taken from the original idea. More often than not it’s missionaries which inspire script creation (as bad as some of you may think Missionary work is, the cree syllabary wouldn’t exist had missionaries not visited the Cree).
There may be many scripts we don’t even know exist. But, of the scripts that are invented in whole, we have:
Sophisticated Grammatogenies
These are scripts created by people already literate in a writing system, but usually created for languages who have no writing system of their own:
The pollard Script
The Fraser Script
Fictional scripts (Klingon)
Unsophisticated Grammatogenies (usually syllabaries, consonant vowel syllables, and random order of syllables)
Bamum script
alaska script
Ndjuka script
Caroline islands script
Cherokee script
The more widely used invented scripts are:
Cherokee
N’ko
Vai
Cree (which spawned inuit, and sub-arctic athabaskan, as well as algonquin syllabaries)
Munda language scripts (Sorang Sompeng, Ol Cemet’, Ho)
Pahawh Hmong script
Korean
Lepcha
Phags Pa
I’m probably missing a few.
The oldest known invented script Old Persian Cuneiform, the best documented is Korean. The most celebrated is Cherokee. One school of thought even says that tibetan was created rather than evolved.
As for not adding in diacritics, as Hari said, don’t be so sure. It’s far easier to add in diacritics to keep form than it is to add in bunches of digraphs to indicate a single sound. Many of the slavic languages that use the Latin Alphabet do this (Polish uses a barred L for the “w” sound). Umlauts also have an important function, to indicate functional sound changes.
Take for instance Phajhauj Hmoob, where the extra letters indicate tones. Without tones it would be Pa Hau Hmong. Pinyin effectively uses diacritics to shorten word length (and Pinyin was created in the 20th century). Diacritics are extremely handy, which is why when alphabets for languages so distinct from the Western European Languages use them, they go for diacritics and not often digraphs (two letters for one sound). Vietnamese is similar (it would be extremely inefficient to indicate tones and vowel quality using other letters like Hmong in Latin Script does).