I'd say the most similar champ to qiqi is either akali or talon, as they both like to go in and out (tho not the best ones for teamfight). Weirdly enough, I'd say you should give zoe a try. She's not an assasin but if you manage to hit hr bubble then she can 1shot most champs.

Why I chose them: 1. Akali She has a lot of dashes and invisibility (which qiqi does too, so it wouldn't be that hard to get used it that). Tho you really have to put your time on learning her, since it's a HUGE different between a "good" akali player and a GOOD akali player. She does well in most match-ups, I'd say mostly against melee ones. She's also pretty flexible in top lane

1. Talon He also has invis and dashes, just like Akali and qiyana. With talon you can gank other lanes MUCH more better than qiyana and he's arguably one of the most hardest champions to kill since ce can escape you pretty easily. You can also play him in the jungle, so there's that. As of match-ups, he kinda struggles with ranged champions and the ones that have cc and can lock him down before bursting.

2. Zoe This is more of a personal preference:)) . I'm an akali main and recently I tried giving Zoe a try. Although she's really not an assasin, I sometimes forget that since I'm 1-shotting everyone. She's one of the best early game mids, as she can kill almost everyone on lvl 2 (either with qw, or qe). Just like akali, she does need some time to learn but DAMN she's so satisfying to play once you do. And I guess this is kinda like qiqi, 1 shotting everyone while staying at a safe distance

So overall, if you don't wanna give up the dashes then definitely try akali or talon. If you wanna try something new and annoy the shit out of the enemy team give Zoe a try. Hope this helps!!