Clearly one argument against expanding beyond ASCII would be disagreement over which letters to include! My preference would be where possible to stick to non‐precombined characters; thus we'd both be ok with ⟨Þ⟩ or ⟨Æ⟩, but I'd avoid ⟨Š⟩ and ⟨Ä⟩ whereas you'd (presumably) take exception to ⟨Ƿ⟩ or ⟨Ꞵ⟩ (assuming those even show up for you).
When Ralph Betza first came up with his funny notation, I believe the purpose was for communicating piece movement capabilities in a brief notation that could be easily read by another person. In contrast, H. G. Muller's xBetza notation has become more of a programming language, and it already comes across as an obfuscated one at that. Personally, I don't use funny notation or xBetza, because it is already too complicated. I prefer to use English for communicating with other people or a full-fledged programming language, such as GAME Code, for programming how a piece moves. One question you should ask yourself is why you want to expand xBetza. If it's to easily communicate more types of piece movement to people, I think it's going to reach a smaller and smaller audience as you add more characters to it. If it is to more easily program types of movement not already supported, I think it would work better to allow the use of longer names through some kind of bracketing, sort of like Game Courier's FEN allows longer names within braces.
When Ralph Betza first came up with his funny notation, I believe the purpose was for communicating piece movement capabilities in a brief notation that could be easily read by another person. In contrast, H. G. Muller's xBetza notation has become more of a programming language, and it already comes across as an obfuscated one at that. Personally, I don't use funny notation or xBetza, because it is already too complicated. I prefer to use English for communicating with other people or a full-fledged programming language, such as GAME Code, for programming how a piece moves. One question you should ask yourself is why you want to expand xBetza. If it's to easily communicate more types of piece movement to people, I think it's going to reach a smaller and smaller audience as you add more characters to it. If it is to more easily program types of movement not already supported, I think it would work better to allow the use of longer names through some kind of bracketing, sort of like Game Courier's FEN allows longer names within braces.