I would like to recommend Sion

I rally appreciate this guide. Although not lvl 30, reading up on this makes me want to try ranked games out when i do

That's a great point, Ill start working on those sections today. They are areas where I see people mess up on or don't know how to handle all the time.

Great guide, however what should you do when it comes to late game. For example, level 16+ and no inhibitor towers are down. Also if you're behind in a 3v3 game without taking any turrets down.

If your opponent plays passively then that will work. The problem is, most don't. I rarely ever have a game where there is not a level 2 fight. Then a 3,4,5 and 6 fight after that. Early game is the king in this map. It isn't all that matters, but it plays a major part in it. Obviously different champs will have different build paths to take, however, in general, you want to be rushing either a Blackfire Torch (champions like Syndra/ Annie) or a Wooglet's Witchcap (most other mages) because of the damage and the extra survivability from its active. Morellonomicon has gotten some popularity as of late as well because of its strong stats, and cheap build path. Rod of Ages, although not a bad item, won't help you nearly as much most of the time.

I would disagree with you that mages need to be relevant early game. I run Greater Seal of Scaling Armor, Greater Glyph of Scaling Magic Resist, and Greater Quintessence of Movement Speed on Anivia (who already has a weak early game). You neglected to mention something very important to TT: Levels only require 80% of the experience compared to SR. When you add this to the fact that lanes are much shorter, so minion waves hit the lane more frequently, you have a massive flow of exp coming your way.

I'd actually say to focus on the opposite as a mage: just focus on getting fat as quickly as you can. A single Doran's Ring is enough to keep you relevant (although not a monster). I take Clarity on Anivia, and literally ult almost every single minion wave. With the increased mana regen on this map, you don't even need Unholy Grail. Between Rod of Ages, Clarity, and the increased mana regen, there are few mana problems, and you start going for damage much earlier.