Not a fan of starting without some anchor text or characters. If we break it down, there's a name and a parameter part. You currently have syntax for tags with parameters like so:
Extrapolate a bit without conflicting too much with Markdown:
##Do Next Week(@due >[d] today and @due <[d] end of next week)
That would also stand out a bit better too. It would look like a header with Markdown parsing so I don't think it's too terrible.
I'd even be good with
@@Do Next Week(@due >[d] today and @due <[d] end of next week) but that might make parsing harder but it is supported with other apps (not recognized as a valid tag).
That seems more inline with Taskpaper format since it resembles the tag/attribute relationship.