Something that popped into my head recently. Hasn't been properly clarified anywhere, so wanted to write up something. Why? This is why. On all seriousness, if implemented this would be for the edge cases of when the Japanese make up their own words and you can't figure out how to Americanise™ it.
Need help on wording, as it's a bit long, but I wanted to make it as clear as possible.
When romanising the Japanese chouonpu (aka. the long vowel symbol) yourself and no official romanisation is available, the vowel before chouonpu should be repeated. For example, オー and イー become oo and ii respectively. This does not apply for loan words from other languages or when existing Japanese words are written with chouonpu, such as せんせー being a variation of せんせい.
Need help on wording, as it's a bit long, but I wanted to make it as clear as possible.