Phew! Let me have a go.
Using your very helpful map, the plan for the circuit is as follows:
1) New pits and paddock complex to be built on the inside of Starkeys Straight
2) The Esses will be bypassed, creating one long curved straight from Coppice down to the Melbourne Hairpin
3) Coming out of the Melbourne hairpin, instead of turning left at Goddards to join the current start-finish straight, the track will go straight on for a short distance, past where the current Esses are, then will curve around to the left in a long gentle curve back to rejoin the Wheatcroft Straight roughly where Goddards is now - essentially a short infield loop.
Mercifully the section from Redgate to Coppice will remain unchanged.
As for the length issue, the only criterion from the FIA is that it must be 4km at least, which the current Donington only just manages. Why would you lengthen it? Well, traffic is an issue on short circuits, particularly in qualifying, blocking becomes a severe problem, as well as cars being repeatedly lapped during the race. The current F1 cars could lap Donington in about 1m06 or 1m07, which is a pretty short time really.
I do agree with you about the lack of variety in circuit length in F1 now though. The big old road courses were too difficult to marshal effectively, problematic for TV and often a bit dangerous, hence the old Nurburgring, Spa and Hockenheim have all been shortened.