Aurelion Sol: You may want to hold off buying this guy, because think about it: if you buy him on release, you're gonna be wasting a lot of potential IP, and even then you might not get to play/main him because he may be permabanned. He hasn't been balanced yet, either, so he may be released in a weakened state. Also, Aurelion Sol is not very simply to play, as he focuses on zoning and keeping enemies a certain distance away, a mechanic you'll get used to as you play LoL more.
Gnar is, quite literally, one of my favorite champions to play, having extreme outplay potential and relevance and is just overall fun in every stage of the game. However, Gnar is mechanically intensive, on par with Orianna in the sheer amount of micro-managing you have to do in Mini-Gnar when kiting a champion, although he isn't as much in Mega Gnar, since you just slam everything in a wall; however, that still requires good positioning. If you're playing for fun, I would pick Gnar up, but be warned that he is NOT easy to play. Especially if you're taking Gnar jungle, you'll have to have runes if you don't already, otherwise you'll be steamrolled by jungle creeps and die in your third clear. Just be careful, since you're barely starting the game.
Tahm Kench is one of the most cancerous champions in League, but I guess it's okay to play him. Tahm is very straightforward, not requiring much mechanical skill but more important in terms of decision making. You have to decide whether to swallow a person or stun them; often times, the right answer is the one you didn't choose. Utilizing his ult also requires a lot of strategic planning if you're going support. However, for a new player like you, Tahm shouldn't be too hard to play; he's very fun when you save your teammates (or screw them over :P).
Renekton is a lane bully, and requires very little skill to play, and is, just like Tahm Kench, more of decision making in terms of what you use your fury for. Renekton is very similar to Tahm Kench except for the fact that he has a dash and does physical damage rather than magical damage.
Rengar is the "trade one champion for one-three other" champion. Assuming you know how to build Rengar, Rengar should be very easy to play, but I consider him one of those "anti-fun" champions, so please don't play him.
Nautilus, while good in ARAM, is food in SR. Naut is one of those CC tanks that lock onto a target and CC them too death, while your teammates kill them off slowly. Naut isn't very good of a champion, however... he has poor scalings and often is a worse Leona. I wouldn't recommend him at all.
Zac surely looks cool, and plays very nicely. Zac requires micromanaging, but not as much as Gnar, but certainly more than most champions if you want to get him to max efficiency. Zac is resourceless (HP doesn't really count, since your passive heals you up more than the cost), which is good if you're starting out, since you don't have to worry much about mana and more about hitting your spells. Zac, however, may feel clunky at times, so beware.
Looking at the champions that you already own, I can infer that you enjoy playing bruisers (Malphite/Gragas, and maybe Tankyi). As such, I recommend these champions to add to the roster of possible champions to play:
ChoCho is very fun champion that you can take to the top lane. You usually don't have to worry about harass and can make the enemy cry because of your bullsh*t mechanics. But overall, he fits into the bruiser role, and it's worth checking him out. He's easy to play and pick up. He's superb at jungling too, because of his passive and his Feast.
Dunkmaster is also a very nice bruiser that you can take both into the top lane to make the enemy cry or the jungle to make their team cry. He suffers from lack of mobility, but that's mainly his only weakness, so he should be fun enough for you to play.
Also bruisers, and sporting more mobility than usual. If you like to go fast, then these are most likely the champions for you.
I hoped I helped you, and sorry for posting this wall of text.
sejuani, also tank, full of aoe.
amumu easy to use, land your hook, use your ult, everyone stunned.
warwick, similar to yi, has insane sustain, can stun a champ with his ult (kinda like malzahar 's), can track down foes with low hp.
