Things that would need clarification imo (regarding Japanese):The case of wi [ゐ/ヰ] we [ゑ/ヱ] and wo [ヲ] is a difficult one, as they are no longer 'proper usage' in japanese. They were fazed out because they no longer served a distinct difference to other kana. ゐ/ヰ slowly started being just pronounced い [i] and served no spoken difference, same goes for the others. へ [he] and を [wo] being pronounced as え [e] and お [o] when used in particle form is a modern day hold over from the same sort of thing. Which is what makes the romanization case not entirely cut and dry.
Particles - は, へ and を
は (ha), when used as a particle is pronounced wa. I've seen both romanizations in osu! map names.
へ (he), same thing. Pronounced e as a particle.
を (wo) becomes o.
This may sound inconsistent, but I believe wa, he and wo should always be used.
Then there are two more issues that I'd like to give an example for.
妖精帝國 - 空想メソロギヰ
In the ranked maps, this has been romanized as "Yousei Teikoku - Kuusou Mesorogiwi"
Issue #1: The romanization of メソロギヰ
This is a stylized spelling of ミソロジー / misorojii / Mythology; the outdated character ヰ (wi) is basically pronounced as i. Do you go for the literal romanization Mesorogiwi or the English equivalent Mythology?
Issue #2: Official vs literal romanization of artist names like 妖精帝國
Here, an official translated version of the name actually exists: Das Feenreich.
However, all of osu! is completely used to the literal romanization of Yousei Teikoku. Forcing the "official" name would only add to the confusion in cases like this.
I think any case of the obsolete kanas wi [ゐ/ヰ] we [ゑ/ヱ] wo [ヲ] should be made an exception to a 1:1 rule and to be romanized as their pronounced forms, as they are NEVER spoken in their original sound, and romanizing them 1:1 to their original classical sound messes up the actual speaking of the word it is used in, as they aren't spoken that way. (which is why they are obsolete characters)
ゐ/ヰ [wi] = i
ゑ/ヱ [we] = e
ヲ [wo] = o
As for the current particles that have a similar case, は [ha] へ [he] を [wo] I think it should be the same. Ha [は] when spoken as a particle sounds like wa [わ] but is not WRITTEN as such. Something like 「あなたは」 is written 'anata HA' but said as 'anata WA'. I believe in romanization it's important to get across what is actually said, as in my case for the romanization of the obsolete kanas above. So ha [は] in it's particle form should be romanized into what is actually said with 'wa' and the particles へ [he] and を [wo] should be similarly contracted into e and o.
は [ha, particle form] = wa
へ [he, particle form] = e
を [wo, particle form] = o
This basically accounts for ALL the exceptions in Japanese that aren't pronounced or used in a straightforward way.