Hello,
I think a variation of the UK's ISA would be a very good thing - but I'd be looking for a way to incentive those with less, to participate.
Maybe only make it available to those with an income of less than "X" per annum (this could be easily checked, through the tax or social welfare records). If a person later goes above the income threshold, it gets frozen, so can't accept new deposits.
Perhaps offer a bonus for those on low / no incomes, not dissimilar to the bonus paid on the old SSIA accounts (but only to be paid to those who meet the low income criteria).
Like Mr. Burgess, I think it also needs to be keep fairly simple, and would be happy to see people allowed to withdraw funds periodically, and then top back up, to whatever the maximum limit was.