It would not be wise to do this.
"Brainwallet" software is completely non-standardized, there's no guarantee that any two implementations handle the encoding of characters in the same way. Addition of UTF8/UTF16 these characters is also likely to be incredibly confusing, did you mean Tooxy or Тооху? Those strings look the same but one is a completely different set of Unicode characters (and make two entirely different keys).
Even with the addition of non-standard characters it's unlikely any particular human generated string will have enough entropy to avoid attack. Staying far away from the brainwallet concept entirely would be most sensible, it's extremely broken and will likely lead to a loss of money either through theft or faulty software.