Fine-grained control over various GC phases and decisions (such as the level of parallelism, max pause times). Until Go 1.19, it was even missing a max memory limit, and even now it only has a soft memory limit.

Additionally, of course it would be nice to have more GC options, such as choosing between a throughput-oriented GC design and a latency-oriented one, having the option of a compacting GC of some kind to avoid fragmentation, or even having a real time GC option.

Go has chosen a very old-fashioned GC design with very few tuning parameters possible, but even so it only exposes a very basic form of control.