You could change the Ctrl+T hotkey to run this:
The New Tab
+ button at the end of the tab bar can't be modified, on the other hand. A script could set focus to the location bar whenever a new tab opens, but I'm not sure that would work too well as it would also happen when tabs were opened via other methods (e.g. sending Find results into a new tab).
You could probably do the script approach if you have Opus set to open empty tabs when opening new ones, as the script could then only change focus if the new tab was empty. in that case it should be a fairly simple script, although not as simple as the Ctrl+T change.