A big goodbye could be on the way, and we may want to prepare ourselves.
Roy is the one that got away for her and he clearly reciprocates her feelings.
If Thea leaves Star City, it will obviously be sad for Team Arrow as well as for fans who have gotten used to having Willa Holland as a regular for the past six seasons of the show, but running away with Roy is possibly the closest she could possibly get to a happily-ever-after, and they both certainly deserve a happy ending.
Thea was taken aback that her protective big brother was giving her the green light (not that she needed it) to join Roy in the fugitive life, but the stability of his personal life after finally marrying Felicity seems to have given Oliver some perspective, and he acknowledged that Thea might need more from her life than what she could get in Star City.