This has been suggested, but until we can buy fractional ETFs I don’t think its do-able. I believe it’s a planned feature though.
Incidentally there are some good guides on here in the Passive Investors thread to creating your own pots which mimic all-world ETFs and allow you to skew one way or another to emphasise emerging markets it whatever, though you will have to buy the individual ETFs yourself to maintain the % balance.