Pan-democrats want the 35 functional constituency seats in the Legislative Council sharply reduced to 20 in 2016 to pave the way for a fully democratic legislature in 2020.
The proposal from the Alliance for True Democracy has put the next Legco poll in the spotlight at a time when political debate is being dominated by the electoral method for choosing the chief executive in 2017.
The grouping of 26 out of 27 pan-democratic lawmakers said all functional constituencies should be abolished as soon as possible.
"We understand that it may not be easy to realise that idealistic scenario by 2016, so we are ready to offer a transitional plan," convenor Professor Joseph Cheng Yu-shek said.
Under the plan, to be tabled to Beijing officials during the lawmakers' visit to Shanghai next month, the 35 geographical-constituency seats would be retained. But the indirectly elected trade-based seats would be cut to 20 and merged into three general sectors - professional, commercial and socio-political - to dilute each group's interests. The five district council (second) functional seats - the so-called "super seats" elected by 3.2 million voters who do not get a vote in any other functional constituency - would be among the 15 seats abolished.
The scrapped seats would then be replaced by directly elected seats in which all non- functional-constituency voters can pick their lawmakers by proportional representation, with the whole of Hong Kong as a single constituency, implying that all voters would enjoy two votes.