GIST. Ranged engage, if missed don't need to commit, against melee heavy team you can ult on top of them but be careful of being bursted.

Get mobility so you get places quicker or maybe sorcs depends, i don't feel like sorcs is as amazing as mobility but mobility got nerfed recently anyway.

You can get any of these and they should be good, it really depends on what your team needs.
Buy homeguards, at some point during mid game, it allows you to get to places fast.
Get randuins if they're melee heavy AND physical damage. it works well to be tanky too and getting your cc in with ult just in case you don't zhonya in time. It slows them down which helps keep them in leash range.
Frozen heart against auto attack reliant teams.
Twin shadows is good for chasing and disengaging as well.
Morellonomicon can work if they have a lot of healers and you don't have enough cdr.
If you're super ahead of course building full damage can be effective too.
Locket is good for shield bating too, also against AP heavy teams.

-A when trading health, you should always aim for equal at worst. Never take free harass if possible.
-Always try be in the most threatening position while avoiding engages/harass
-Clear waves after forcing them to back, then back.
-Ward when you've pushed them back, that way you won't or shouldn't be cut off when coming back.
-When purple side you can ward the river bush from the tribush near blue.
-When blue side and pushed in you should walk around near the golem brush and ward the tribrush from there.
-Always prefer harassing ADC over support, unless the ADC can heal significantly more than what youre going to do (e.g. they have BT) or you can easily and safely get harass on support when you don't need to save spells for possible engages
-If you start harassing one of them, but the other moves closer, re-evaluate which one you should be targetting. This can be used to bait the adc into a bad position by harssing the support once, then changing to him.
-If going all-in, exhaust the highest threat, unless exhaust has to be used to secure a kill where you won't die from the other threat.
-Trading your life to their support is fine if your adc kills their adc, in almost all cases (Excluding cases where you're worth a lot and/or they're worth too little).

-Make sure dragon and baron are warded nearly 100% of the time.
-When your bot turret has died, you are not nearly as required to be at bot turret.
-When ahead, evaluate your roaming potential. When the mid laner has no flash or is going to bot through river, you can cut them off but ofcourse, don't forget to call your midlane and say what you're doing.
-Communication is key, if you're intending to ward baron, it's not a solo mission, if they're posing for it make sure that the team knows that you want to ward baron and you need help. Everyone has to apply pressure to do this safely.
-If you have ignite and they have a mundo make sure to ignite him as soon as he ults. This reduces his healing rate!
-If you have exhaust it's not always necessary to exhaust their ad, using your exhaust to peel for OUR ad can be better sometimes, make sure to evaluate the situation.
-Bindings can be used for peel instead of engage, your ult likewise.