It seems we need a new method in Type_handler, like this:

virtual const *Type_handler tmp_table_handler(const Type_std_attributes&,

                                              const Type_ext_attrubutes) const;

The logic for the traditional Item types is hard coded in Item::create_tmp_field() in the switch by cmp_type(). It performs conversion to a super-type which has the same cmp_type(), with a special code for GEOMETRY.

Note, conversion to a super-type can often be excessive. For example, for INT-alike types we create Field_long or Field_longlong. But under certain circumstances smaller types should be enough (e.g. when the result is not exposed as a field of a permanent table). An additional "what_for" parameter will possibly be needed for tmp_table_handler().

For example, if data type of "this" is MYSQL_TYPE_SHORT, and the field is needed only for internal purposes and is know not to overflow, we could create Field_short instead of Field_long.

In order to do this, the code needs to be fixed first to make sure that type_handler()/field_type() are always in sync with max_length for all Item types, which now is not always true.

Update

After this commit:

commit 109bc47084c72164130fc2e735ffe371713ede43

Date:   Thu May 25 15:15:39 2017 +0400

    Fixing a few data type related problems: MDEV-12875, MDEV-12886, MDEV-12916

    This is a joint patch fixing the following problems: 

    MDEV-12875 Wrong VIEW column data type for COALESCE(int_column)

    MDEV-12886 Different default for INT and BIGINT column in a VIEW for a SELECT with ROLLUP

    MDEV-12916 Wrong column data type for an INT field of a cursor-anchored ROW variable

this code was moved from Item::create_tmp_field() to Item_sum::create_tmp_field().

So this task becomes lower priority, because only numeric data types are now affected.
Adding string-alike data types like INET6 and UUID do not seem to need this change.

Update#2

As of 2018-06-19, the code of the subject looks like this:

Field *Item_sum::create_tmp_field(bool group, TABLE *table)

{

  Field *UNINIT_VAR(new_field);

  MEM_ROOT *mem_root= table->in_use->mem_root;

  switch (cmp_type()) {

  case REAL_RESULT:

  {

    new_field= new (mem_root)

      Field_double(max_char_length(), maybe_null, &name, decimals, TRUE);

    break;

  }

  case INT_RESULT:

  case TIME_RESULT:

  case DECIMAL_RESULT:

  case STRING_RESULT:

    new_field= tmp_table_field_from_field_type(table);

    break;

  case ROW_RESULT:

    // This case should never be choosen

    DBUG_ASSERT(0);

    new_field= 0;

    break;

  }

  if (new_field)

    new_field->init(table);

  return new_field;

}

There is no special GEOMETRY code any more.
There is only a piece of special code for REAL_RESULT.