The Welsh Parliament should have 20 to 30 new politicians, a committee has recommended.
The Senedd electoral reform committee is made up of Labour and Plaid Cymru Members of the Senedd (MSs) but was boycotted by the Welsh Conservatives and Brexit Party.
It wants a more proportional system at the 2026 elections. Plaid was the only large party to back the findings.
Committee chairwoman Dawn Bowden said more needed doing to build a consensus.
If it went ahead, it is estimated an extra 30 MSs would cost about £12m a year.
The Welsh Conservatives said they saw no reason to change the Welsh Parliament's voting system, while Welsh Labour did not comment on the findings.
There have been calls to expand the size of the 60-member Welsh Parliament - which was known as the National Assembly for Wales until May 2020 - for some time.
An increasing list of law-making and taxation powers devolved to Cardiff Bay led to an expert panel in 2017 recommending an extra 20-30 members.
But the idea was not endorsed by Welsh Labour at the time and unlike other changes to the institution, such as votes at 16, it was left to the next Senedd term to decide.
The Senedd electoral reform committee was formed in an attempt to find a cross-party approach ahead of the next election.
While the Welsh Conservatives declined to take part from the outset, the Brexit Party did initially join but committee member David Rowlands left over a row with Ms Bowden.
Ms Bowden said she was "very frustrated" she was not delivering a cross-party report and it was a "source of disappointment" the Conservatives did not take part.
She said she would like to see at least key principles of the recommendations, "if not in their entirety", to be in Labour's manifesto for the next election.
The support of at least 40 MSs would be needed to pass legislation if the recommendations were agreed.
What does the committee say?
The committee recommends legislation is introduced after the 2021 election for MSs to be elected using the single transferable vote electoral system, which asks voters to rank individual candidates and aims to return a proportional number of politicians according to who voters back.
Currently, MSs are elected through a mix of first past the post and party lists.
Committee members backed the expert panel's call for a Senedd of between 80 and 90 members - with effect from the 2026 election - saying the 60-member Senedd is "too small to carry out its scrutiny, legislative and representative functions".
Ms Bowden said: "A larger Senedd would be cost-effective, as members would be better able to hold the Welsh government to account for its spending and decisions, and pass better legislation, as well as helping people across Wales with their problems.
"It's not about 'can we afford to do this?', it's 'can we not afford to do this?'"
The report also called for political parties to "collect, anonymise and publish" data about the diversity of candidates.
What did the Welsh parties say?
Plaid Cymru Leader Adam Price said: "Our parliament is too small and that represents a big danger to the health of our democracy."
The Brexit Party said the report had "no legitimacy" and strongly opposed "any increase in the size of the Welsh Parliament."
Paul Davies, the leader of the Welsh Conservatives' in the Senedd, said: "This is a report which represents the views of just two parties in the Welsh Parliament and currently there is no public appetite for an increase in the number of politicians."
Labour said its full focus was on "a careful and cautious response to the pandemic".
Jess Blair, Director of Electoral Reform Society Cymru, accused politicians of "choosing to play politics with the issue, avoiding making the crucial decision that would give the Welsh people the voice they deserve".
A Welsh Government spokesman said: "This committee report makes important recommendations for the future of the Senedd, which will need to be carefully considered by all political parties and will be for the next Welsh Government and Senedd to take forward in terms of legislation."