In the following examples, I am using '|' to denote the insertion point.

When entering an opening bracket as follows:

 func_name(|

...the widget should insert a matching closing brace, while leaving the
insertion point INSIDE the bracket pair so as to allow specification of parameters:

 func_name(foo, bar|)

Once parameters have been specified, pressing enter should move the insertion
point out of the bracket pair:

 func_name(foo, bar)|

Pressing enter again should move the insertion point to the new line as normal.

 func_name(foo, bar)
 |

In the case of multi-line blocks, typing the opening brace and pressing enter:

 func_name(foo, bar) {
 |

... should result in a matching closing brace being inserted on the line
following the line of insertion, as well as auto-indentation:

 func_name(foo, bar) {
   |
 }

Perhaps there could be a keyboard shortcut named Next Block or something similar
that would move the insertion point out of the current block and into the next
available one:

 func_name(foo, bar) {

 }
 |

Since some languages use whitespace and indentation to define blocks, while
others use "begin...end" or "def funcname...end", there would need to be a way
to define the beginning and end of single-line and multi-line bracket/brace
pairs. The same architecture might be used for XML opening/closing tag and
processing instruction completion, as well as multiline comment completion
(think "/*" and "*/", or "<!--" and "-->").

Also, logic structures like "if...else...end" could benefit.