framework best practices for parallel execution support

[AVI]

I have a good selenium automation framework, which is used across different projects without any issue if I run tests in single thread. 

Recently I tried to enhance my framework to enable parallel executions, but I am finding it difficult. So far I am able to resolve different issues I encountered (listed below), but getting more issues as I progress. I am thinking to write a new framework considering concurrency at every steps. Please can someone suggest best practices (do's and dont's) to be followed.

Few of the issues I encountered and resolved so far: 

a) Static Webdriver instance of Browser factory class did not work 

  --- adopted Factory design pattern with ThreadLocal, and it worked

b) Multiple instances of chrome were not working if used certain with chrome options 

  -- disabled those options

c) Was creating a new pdf file (after deleting the old one) to upload for a given requirement. Encountered a situation where a thread deleted a file created by another thread before the upload. 

  -- removing old files in at the end (@AfterSuite) method

there are more similar issues....