The government says it will think "very seriously" about introducing a 4,000-word essay to allow universities to choose between top A-level students.
Colchester twins, Yan Ling Li and Yan Yi Li, got straight A grades
Over-subscribed universities complain the current system does not allow them to pick out the best candidates.
This year, the number of entries gaining an A grade rose from 21.6% to 22.4%.
David Miliband told BBC Radio 4's Today programme an extended essay was one of several "options" for testing skills.
'Demanding obstacle course'
Under the plan, universities would be given copies of the essay to assess the intellectual skills of sixth-formers.
FINDING THE BEST STUDENTS?
Extended 'super essay' could be given to universities
Students' individual marks could be published, rather than an overall grade
The grades of each module could be published, rather than a single overall grade
Most students apply for university places before they sit their A-levels.
Admissions tutors claim this gives too little information to differentiate between them.
The extended essay idea is part of a proposed diploma scheme to replace the current GCSE and A-level system.
Its author, the former chief inspector of schools, Mike Tomlinson, will present his final recommendations to the government in the autumn.
Mr Miliband said it was important to "stretch the most able candidates and recognise their achievements".
However, the 2-3% of young people who gained three grade As at A-level had already gone through "an extremely demanding obstacle course".
Some subjects have a particularly high number of students achieving A grades at A-level.
In maths this year the figure is 37.9% and in French it is 33.4%.
Delyth Chambers, director of admissions at Birmingham University, said: "The difficulty arises earlier in the year, not at results time. We are basing our offers on future performance."
Universities want ways to distinguish between A grade students
Some universities have already started to look beyond A-levels when recruiting.
Several hold their own admissions tests, looking at the skills students have. Some are subject-based, others are more general.
A few universities set their own full exams, although concerns have been raised that this would increase student workload.
Another option suggested, is for all students to apply after A-levels and for universities to be given a full numerical breakdown of their performances - with specific percentages rather than an overall grade.
This would be intended to help differentiate more accurately between students who have an A grade.
The overall percentage, known as the Uniform Mark, is already given to students with their A-level grade.
Another option would be to give the grades for all A-level modules, rather than a single combined grade. While there are a growing number of students scoring an overall A grade - only a very small number gain an A in every exam module.
Examining boards have also started offering AE, or Advanced Extension, levels. These assess leading candidates' broader knowledge and awareness of A-level subjects. But there were only 7,246 such exam entries this year.