There is no single answer and as the percentage of the interconnection supply to the distribution load increases the answers change (yes, I did say answers).
First is inertia, which mostly in the future will be supported not by generation, but by load (and that has other larger issues). Motors in Airconditioning units and other loads will help maintain inertia.
Next is protection, which likely will have to change two or three times a day in the future instead of changing once or twice a decade.
Third is power angle (VARs), which will come from the interconnections, and some level of capacitor banks or other devices (eventually we will have solid state transformers that may solve this – but when is a HUGE question).
Fourth is voltage management, too much is some places and not enough in others – this one is more difficult to answer because interconnections have local impacts on voltage, that can not always be handled by the substation.
Fifth is phase imbalance, most smaller interconnections will be single phase. Get a subdivision all with solar on a lateral, and all the solar will be on one phase, which can have negative impacts on three phase customers.
Six is overall energy availability, a cloud transient (if sudden enough and dark enough) could cause massive power inrushes to the substation which may trip protection and black out the circuit/substation.
There are more but this should give you an idea of the complexity.
Most distribution does not have many remote controls, and the most common is “turn the power off”
The idea of an Advanced Distribution Management System that is going to eventually give visibility and control to the operator (note many distribution systems do not have operators, just dispatchers).
To make much of this work, we will have to deal with the idea of who is in control and what can be controlled. So far most the interconnections are uncontrolled and unmonitored.
We are at the starting line, even in California and Hawaii and several blackouts may be required to change current practices.