There's already a basic integration but it's mainly just to push to RTM from HA. Creating a sync function and pulling RTM into HA lists would open up a lot of smart home doors.
Their ToDo function is HA is ok(ish) but the automation capability is so customisable and using it for 'storing' common tasks like standard shopping items would help declutter the actual workspace.
Plus it's open source and I feel like you all share a similar target audience, the set up felt really similar and seeing you have something for HA was what pushed me into using RTM.