So i think I've been getting pretty solid in solo queue with both 
and 
in the mid lane, but I want to do more than just be solid with only two champions. All I want to know is some champions that are not too difficult but viable in the mid lane. Cost is not an issue, btw.
Can you hit skill-shots? If so, give 
a shot. I've been maining her for about a year with great success (plus she's really powerful right now). She has very few counters (I can play her against any champ) she has a solid lane phase, and she's a terror in teamfights (CC, outstanding range, high damage). You'll have to get used to standing behind your team though, she's kinda squishy lol
Also, I've found that 's playstyle is similar to 
, and if you can play one you should be able to play the other. 's skillshots are more difficult to learn to aim, but also more difficult to dodge (he's a bit of an off-pick, and I've found that a lot of people have no idea how to play against him). He's a little more squishy, but his damage is slightly higher. I personally prefer , but this guy can be a pain to play against.
I think I'd say has a slight laning edge, but has a considerable edge in teamfights.
Also, if you don't know about it, I'd recommend keeping an eye on this list http://www.nerfplz.com/2015/09/2015-champion-tier-list-solo-queue_20.html and the site its on in gerenal, it has some pretty useful advice.
Honestly I think Annie is the easiest mid lane. Fizz may be a bit too difficult, when I was level 15 I mained fizz and was absolute garbage at him even before the nerf.
Other champs: 
(build Rod of Ages, it would be an AP tank mid)
are other good picks.
This is my opinion, different champs usually have different play styles. Some are really aggressive and others are really passive, others just poke the champs from a distance. What play style do you usually have?
Depends on style, you're still low summoner level so I'd say check out different play style champs unless you're sure you are 100% an assassin fan and you really just want to play burst/assassin champs.
Assassins to try:
Ahri has lots of movement like fizz, brand is kinda similar to annie
Control/pushing/bully champs: Good clear, lots of burst post 6. Stupid sustain, silence to punish mages who need full rotation. 
The god donger (my favorite mid champ) so much harass and such a bully. Great at controlling dragon and pushing lanes fast.
Poke/less mobile ranged mages:
Good luck :D
Malzahar swain ..
i'd be willing to help, I'm silver 5 and started to play only Orianna, Ahri and Azir. if you dont want my help its fine, im not super expierenced.
