Q:

How about Artificial Post Reviewer that finds potential problems in the posts and suggests fixes before users posts?

Why wasn't a thing like this implemented? We could forget about code with no fixed-font, about no 'i's that deserve to be capital and etc. It will lead new users to understand our standard format. While it can vary, but there are things that anyone would agree.
This tool could improve the quality here faster by giving new users jump and it will decrease post bans, and therefore decrease the 'harsh' stigma about Stack Overflow.
My opinion is that it will be worthwhile the cost to implement it.

A:

We do have the community user, who does some auto tasks:
Who is the Community user?

My opinion is that it will be worthwhile the cost to implement it.  

Cost is not the issue. Stack has budgets, and some fantastic developers, but the issue is it's probably impossible.  
When user written code (PHP etc) is involved it becomes impossible to cater for so many potentials.
For example, to know when i needs to be upper case or not, scripts would have to understand context, and this is unreachable where code is concerned as there are too many potential scenarios to cater for.  
Example - a user writes:  

I want to check user login
  My code defines the variable named i ($i) and I want i to be incremented by u (the variable $u I mean)  

Does the scripts  you propose change any case for any of those I's in the above quote? Does it make "u" to "you" - it cannot distinguish unless it knows context.

This could work on non-code Stack sites, so perhaps suggest it on MSE for non-code sites (check it hasn't already, as I'm sure it will have been suggested).  
Edit - in response to your comment
You cannot suggest potential changes to a user unless the suggested changes are in fact an improvement.
However, without more thought and detail in your proposal, currently your "artificial post reviewer" will return a lot of false positives and rubbish.
It's not acceptable for users to rake through such nonsense and silly suggestions.   
What is the point in just swapping one issue for another?
Especially as some users will just accept the suggestions believing that if Stack suggested them then they must surely be ok.  
Anyone who would take the time to review the proposed suggestions would likely be making their question clean anyway, and already doing whatever decent suggestions your system made.
You'd just be annoying them with silly and pointless suggestions.  
Everyone else would just ignore it.  

As it is, your proposal is asking to introduce a problematic system, suggest poor edits to a users question, and as a result would bring issues to the site. Stack wouldn't do such a thing.  
"Take a look at these suggestions, some may be rubbish, ignore those, anything you feel is a good suggestion please change".  
If you can make your proposal show such a system could suggest changes which would be useful and 100% correct, then perhaps it would be considered.  

