Please give this feature @lubos as we need this very much .
How would you propose Manager decide what prefix and suffix to automatically use?
I suppose the invoice number could be made from 3 variable fields, this would cover all the repeated queries for it.
Would be useful to apply it to purchase orders too.
Something like a 3 character prefix (default set by user) the auto numbering as is then a 3 character suffix (default set by user)
Not a show-stopper but does add the flexibility to be adaptable to users existing numbering systems. 
And I can immediately think of several examples users have requested over the last year that wouldn’t be handled by your 3-field scheme. Somewhere, you have to draw the line. Remember, many accounting programs don’t let you customize or edit invoice numbers.
I agree there is a line but this is a topic that keeps coming back.
Most packages allow a preset format of letters and following auto numbers.
Can’t have everything but you won’t know till you ask 
You could actually do this in a custom theme with a conditional test in the section of code that prints the fields array. The entire purpose of custom themes was so people who weren’t satisfied with standard features could add their own, while keeping the program small for the rest of the world.