Choosing a champion to "one-trick" just because you aspire to get a high rank/leave your current rank will only lead to you getting bored of said champion, and end up tanking your ELO by either trying a bunch of new champs in ranked because your new OTP isn't working, or you are just bored.
That being said, I'll just recommend some champions for you to play, as you may want to consider making a champion pool as opposed to finding an "OTP."
LeBlanc, Ahri, and Annie are three ranged champions I think you should look at picking up. They have the same niche as Akali, which is determining and eliminating a priority target in fights. They also have a bit of mobility (Annie excluded, but honestly with your stun and the right summoner spells you shouldn't really die to ganks), and CC in case they get behind which is something Akali can't say.
Vi, Katarina, and Fiora are three melee champions you'd also like. Again, I put emphasis on their mobility because it acts sort of like a safety net for you. These champions also want to get to a high priority target (bar Fiora), and they aren't THAT hard to play.
All in all, keep in mind that you can make any champion work if you put the effort and have the motivation to play them. With this in mind, if you want to keep playing Akali go for it.
Best of luck.