I don't know your playstyle, but if your flexible, here are some strong ones here.
Akali - She's a great assasin, just don't feed pre 6 & then have a blast then. Have a rune page with plenty of AD/AP to activate her passive at level 1 & you'll be a beast
Alistar - (You have to play him very well, btw you should still be able to obtain him for free+ a skin)
Darius - (He's considered god tier, in order to actually prove that you have to play him well, with the upcoming changes to him I wouldn't purchase him. Not for TT)
Diana - Pretty strong jungler, can be a pretty mean laner too
Mundo - Extremely strong jungler here, but your team has to coordinate ganks here, since landing cleavers while in minions can be difficult
Draven - draven is pretty popular at european qualifiers, build some bruiser"ness" on him and he can be a monster here. His chasing power is unreal
Ezreal - Play him well enough, he's great here, bruiser build viable as well
Fizz - Requires a decent team comp to work well, but I've been a victim of those
Garen - Garen good everywhere...
Hecarim - I thought he'd be a great jungler here, boy was I wrong. His dueling potential is really low, he requires a great follow up after his intiation, ex ori ult
Irelia - I personally haven't tried it, but imagine it can work well if your allowed to farm or get kills
Jax - He's a god here, I can solo vilemaw with merc treads & blade of the ruined king only
Jayce - He's pretty good here, chasing potential is great, mean laner, good poker if you have no ranged yet still want another gap closer
Karthus - Hope you have a team to protect you...
Kayle - I've heard rumors of kayle's carrying a game singlely because of her ult. Get a blade of the ruined king, can probably solo vilemaw as well.
Kennen - Needs a good team comp, I won't give any since its a part of mine currently.
Kha'zix - He's mean on 5v5, he's mean here. If he finds you in the jungle alone...better hope you got a flash & his leap is down
Leblanc - She's trash here, she needs to get extremely fed to work well & even then theres much better picks
Lee sin - He's just so mean here, I mean he's just unfairly mean. Another vilemaw solo-er
Master yi - He's pretty fun to play ap here, his nuke potential is high, build ap/mp/cdr & go at it. The new changes coming to him might change that though.
Mordekaiser - Let me just nuke your carry then make that 3v3 into a 4v2. GG
Nidalee - Hope your good at those spears, build her bruiser/ap & win.
Nocture - I feel like he's the same way as hecarim. He sounds good in theory, his ganks are godly, but thats about it. If you need a strong single target intiator, He can be it.
Olaf - He's known for carrying games often
Renekton - He's not a great pick, if jayce/olaf are open or even Vi, I'd pick them over renekton.
Rengar - Gotta love them bushes man...this guy is hilarious to play here if your allowed to farm for a bit
Riven - She's one of those characters on TT where if you get an early lead, & keep that lead, you'll perform well here. If she gets pushed in the dirt...well...have fun
Ryze - Build muramana & win. He's a late game carry, If you have a decent peeler its a blast to just...well blast them
Shaco - He used to be good here, not that great anymore. His ganks are solid, you need to end the game mid-game or else you start to lose hard. You have to build him bruiser like too, which he suffers from
Syndra - Just no...I mean...her team fight potential is high, but she has no escape, she can be dived & kill almost instantly if you want some real damage for her
Teemo - OP mushroooms even more op here...no more wards on TT, but hey shrooms are still allowed.
Tristana - Actually not bad, her escape is fairly good, we usually counter a tristana with darius/alistar/jayce
Tryndamere - He's not as good as he used to be. He's extremely item dependent...aka carry. But I mean play a ranged carry with a better escape. You can build him bruiser but even then its not worth it.
Varus - I just...no. If you are desperately in need of a ranged & need an intiation fine, but hope your good
Veigar - This guy is a blast to play anywhere, TT is no exception. He requires alot of farm, but a couple days ago I was allowed to farm & had full build by 24 min after having a few kills. I had 761 AP & nuked their ziggs instantly with my ult doing 1.35k damage with it. He's farm dependent, but he can destroy games, his cage is very good here as well
Volibear - He's ok, look at renekton for better picks
Warwick - His e is great here, but normally enemy junglers aren't that low, and thats all his e is good for is basically having a personal ward on an enemy. His ult is a great intiation if you need one, but nocture is better IMO
Zed - He's great if you can play him, he's extremely energy starved half the time
Zyra - Pretty item dependent on TT, I'd pick another ap carry
As for your current picks, cass isn't bad, if you can get a full poison comp (teemo/singed/cass) with blackfire you will melt...I mean MELT the enemy team. Its stupid ridiculous how fast you melt them.
Evelynn is trash here, people talk like she's good but the nerfs to her hurt her to much.
Trundle I've actually...never ever played.