Alternatives to y'all?

According to dialect maps created in 2013 by Joshua Katz, the majority people in the Chicago area report that they usually use "you guys" to "describe groups of two or more". In these areas, "guys" is almost always gender neutral as a form of address and is also used without "you", as in "hey guys". (See Is “guy” gender-neutral?)

The Cambridge Online Survey of World Englishes (ongoing) has an interactive map where each response is plotted on the map. Just eyeballing it, again "you guys" is shown as being the most popular in the Chicago area, with "you" also being popular. There were also quite a few people there who reported using "you people" or "all of you".

You may also be interested in checking out the Harvard Dialect Survey (2003).

In any case you should write the best you can, then have your work reviewed/critiqued by someone who speaks that dialect. You've rightly identified one way that you need to change your writing and there may be more that you don't know about.

In the South, the plural of "you" is "you all" or "y'all". In parts of New York, the plural of "you" is "yous". In the rest of the United States, the plural of "you" is "you".

I said to Bob, "You should do this."

I said to Bob and Fred, "You should do this."

I gladly yield if someone is aware of another regional variation on this, or who can say if there is some other word for plural you in other English-speaking countries.

Just to toss in an amusing tidbit: I read a book on grammar once where the writer mentioned that in English we use the same word for singular "you" and plural "you", and that this can create ambiguity. For example, he said, when he was in college, he approached his girlfriend while she was talking with a bunch of other people and asked her to go on a weekend trip with him that he hoped would be romantic. He addressed his invitation to you singular, this one girl, but the others there thought he meant you plural, everyone present. He ended up taking a carload of people on this outing, not at all what he intended.

A lot depends on context, who the speaker is and who they are speaking to.

In a formal context, business or with people you don't know, then second person nominative plural is:

'you'

(that's what the standard is).

If informal, and this is more common nowadays even in formerly ore formal contexts, in the Chicago area.

'you guys' or 'guys'

is the usual. Also 'you all' or 'everyone' will work.

If AAE (mostly black, south side or west),

y'all

is often used.

Most likely you'll want to use 'you', but consider the others depending on context.