So in the arrow season 3 finale we saw Thea finally embrace her training with Merlyn and become speedy ( or red arrow if she chose) and as Oliver left he gave her his blessing and told her to be safe out there. We know for a fact hive will be the big bad of the season 4 and the actress who plays Thea recently said they had big plans for Harley Quinn but had the scrap them because of the suicide squad movie. i read an article on moviepilot and said that Harley could have served a purpose as an enemy but what if they can bring someone else in someone who could match Thea in training and be somewhat of an arch nemesis to her and you maybe asking who could it be well i believe that they could possibly bring in drum roll.......
That's right none other than Cheshire herself. The reason i believe this is because Thea and Cheshire could easily relate to one another both have evil fathers and have very advanced training in hand to hand combat and other weaponry such as Bow and Arrows , Swords , Blades, etc and they have both been in relationship with Roy Harper Cheshire even having his child lian in the comics and it would make for some interesting fights scenes. Also now would be the perfect time to introduce her and help continue Thea's transformation into speedy. they could easily develop a Oliver /Malcolm relationship enemies but in times of need they will work to together to accomplish both their goals but will not have the same amount of hatred for each other and will also keep Thea on her toes when combating her.
One way i would introduce Cheshire is to have her as a hired assassin but let her training have been with someone from outside the league and when team arrow (still without Oliver) starts getting to entangled in HIVE they will send certain operatives to handle them and Cheshire could easily be assigned to take Thea down and during their first encounter the could battle both landing good blows but Thea taking the brunt of the attacks and seeing that she is not skilled enough to defeat her retreats and during the fight Cheshire could even reference Thea's fighting skills as very formidable and they could look at the daredevil vs nobu fight for inspiration . they can continue to have numerous run ins throughout the season and once Cheshire decides that working with HIVE is not right and helps team arrow and fights side by side with Thea
but this is just my theory on how it could all unfold let me know in the comments if you agree.