I think this is a great idea, I also think that the line tasks would be perfect for implementing this as well. Perhaps a secondary option for line tasks called "link tasks" where, if checked, would have this function, and if unchecked the line task would merely be a visual aid.
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Thanks for the great and very helpful program, I'm glad that there is finally something out there that lets me plan visually rather than confining me to columns and rows like trello. I'd be very excited to see if the New Task screen was resize-able, I feel that when I am adding many tasks consecutively a smaller window would be much more ergonomic. Also, the ability to check and uncheck tasks without having to go into them would be absolutely splendid. These two changes alone would go such a long way for me.
Finally, being able to hit Enter to submit a task rather than use the mouse to select the ok button would be fantastic as well.
Congratulations on the excellent software!