Well, adding this to scripting would complicate it quite a lot already. And combining it with a general synchronization is ever more complicated. So it's unlikely, sorry.

But if you have a single folder only, it should not be difficult to implement a PowerShell script using WinSCP .NET assembly to find a new files and then run parallel transfers to fetch them.

See these examples for a start:
https://winscp.net/eng/docs/library_example_parallel_transfers
https://winscp.net/eng/docs/library_example_check_existence_timestamp

I see several older comments that scripting does not support multiple simultaneous connections (queuing). I would like to request consideration of adding this capability.

I have the unfortunate necessity to synchronize a directory with 4.5 million jpg files. If I try and use the GUI it never loads the file list, even after several hours. Using scripting I am able to use the SYNCHRONIZE command successfully, however with only one concurrent transfer it is taking weeks to complete the transfer. I've set the GUI to 9 background transfers and "Transfer on background by default". If the scripting interface would accept those settings it would help optimize my transfer. If the scripting commands could also add OPTION BACKGROUNDTRANSFER 9 that would be even better.