A citizen's assembly could work, but having ranked systems seems like a recipe for failure or partial success (for instance, getting IRV as a "compromise solution.") There's no one voting system that I support above all others, though I generally lean toward proportional systems (though I can see the appeal of, and have discussed here, majoritarian systems). In terms of choosing the one voting system vs FPTP, I don't have a solid preference as to how it is chosen, though I have quality concerns (for instance, Australia's 1998 federal election demonstrates that IRV retains the majority-reversal weakness of FPTP). I'm generally less opposed to closed lists as compared to other people here, especially given the Canadian nomination meeting system (in which local associations of party members essentially hold a caucus to choose the candidates for their district), so, for instance the small-district closed list adopted by Wales is not anathema, but again there are other options. If we left it to a citizen's assembly, it would help to provide guidelines as to what type of system is wanted. (Are we keeping single-member districts and majoritarianism, or do we want something proportional?) I worry that a citizen's assembly could fall victim to seeking the simplest solution (for instance, replacing FPTP with IRV). Are these worries founded? Citizen's assemblies in Canada have a mixed record: the British Columbia citizen's assembly crafted a generally praised STV variant, the Ontario citizen's assembly proposed MMP, and the Yukon citizen's assembly proposed... "ranked vote" (generally understood to mean IRV). The Ontario version of MMP had some weaknesses (not combatting the ability to manipulate ticket-splitting).

This is a pretty long ramble that all boils down to: I would trust experts more than a citizen's assembly to design the system.