With the new dashboard filters, it would be great if we could, for some users or in some cases, increase the number of issues displayed in the "Last 20 issues" block.

It seems that support for a limit is already in the function and we only have to pass that limit. If I add the following line in the file dashboard.php at line 68:
$dashconf['limit'] = 100;

Then I get the desired result (although it's global for all users and hardcoded).