Unicode has defined a mechanism for dealing with the situation, variation selectors. If this mechanism is not appropriate in this particular case, let the UTC come up with another mechanism to meet the user requirement. To define a new set of abstract characters for what are actually glyph variants is to ignore the character-glyph model.
Variation selectors are a tool.
They are most useful if some characters of a writing system or notation show variations that must be preserved in some contexts, but should be ignored in others.
For the mathematical characters, they reflect the inability to get perfect information on whether the shape differences between certain variants of a few symbols are semantically meaningful for a few authors. In theory, they could have been encoded as characters with compatibility equivalents instead, but in the context of how mathematical texts are used, most existing compatibility decompositions produce unintended results...
For the Mongolian characters, they reflect the fact that context analysis is not always perfect, but that it is preferable to have base characters consistently carry their semantic value only, and no information about their positional shape. This makes the model for Mongolian quite a bit more regular for many text processes.
Neither of these cases covers an entire script.
Unlike you, I see no urgent need for Unicode to invent new mechanisms.
Implementers will be glad to know that UTC is not contemplating new mechanisms either -- new mechanisms tend to have a way of impacting, by their mere presence, even implementations that don't want to support them, while new characters don't.
A./
