This confuses the need for a more subtle kind of prose expression on one hand, with the available plain-text punctuation marks on the other.
Rather than inventing a new punctuation mark, I recommend the alternative of choosing different words, or (in many environments) exploiting the existence of different text rendering styles, instead of requiring a new character to add to the vast number of available characters in the Unicode universe. For example, why not use italics for emphasis, for something that doesn't quite merit an exclamation mark?
Or as a last resort, one can use the inverted exclamation mark (¡), which has the advantage of already existing (it's in the extended ASCII character set, hex location a1). This might lead to expressions like "I can't decide how I feel about this!¡!¡".
