Let me keep an asset class at its target weight (e.g. Australian shares at 30%) but choose which ETF inside it receives new contributions, so I can hold one fund as-is and send all new money to another, without selling. This makes it easy to transition between ETFs (for a new lower fee fund etc.) without triggering capital gains tax. Even better if target weights could glide over time (e.g. 70/30 → 50/50) so my contributions do the rebalancing automatically.