Sub NextLetter()
Dim CharRange As Range
Dim iResult As Range
Set CharRange = Range("A1")
Set iResult = Range("A2")
iResult.FormulaR1C1 = "=CHAR(CODE(""" & CharRange.Value & """)+1) "
End Sub
To generate the new character in VBA, look at this code. It increments the first character from each non-numeric string in column A and wites it to the adjacent column B cell.
Code
Sub IncrementAlpha()
Dim c As Range
For Each c In Range(Cells(1, 1), Cells(Rows.Count, 1).End(xlUp))
If Not IsNumeric(Left(c, 1)) Then c.Offset(0, 1) = Chr(Asc(c) + 1)
Next c
End Sub