Hmm…

Is the incorporated Roman numeral IX larger so it doesn't read as "Resident Evil X"? In which case, I would just use an Arabic numeral 9.

Do you think Capcom kicks itself for missing the opportunity to incorporate a Roman number VI for RE6? It's the cleanest incorporation.

Which came first: The decision to set RE8 in a village, or a way to incorporate a Roman numeral VIII in its title?

Capcom also could have split the L in "Evil" into two Roman numeral Is, à la Scream VI:

What words feature "IX" in them?

Resident Evil Helix.