W przyszłym roku cena gazu może pójść w górę.

It is in the locative case.

• The preposition w often means in when talking about time or place.
• After w in this meaning, Polish usually uses the locative.
• So:
  • rok = year
  • w roku = in the year
  • w przyszłym roku = in the next year / next year

The adjective also changes to match the noun:

• przyszły rok = next year
• w przyszłym roku = in the next year

So this is a very common time expression in Polish.

Because rok changes form in the locative singular.

Its basic form is:

• rok = year

But after w meaning in, it becomes:

• w roku = in the year

This is one of those noun forms you simply get used to through exposure. So:

• przyszły rok = next year
• w przyszłym roku = next year / in the next year

Because Polish often uses the genitive case where English uses of.

So:

• cena = price
• gaz = gas
• gazu = of gas

Therefore:

• cena gazu = the price of gas

This is a very common pattern in Polish:

• kolor domu = the color of the house
• koniec filmu = the end of the film
• cena chleba = the price of bread

Może is the 3rd person singular form of móc, which means can / may / might / be able to.

Here it means something like:

• may
• might
• could

It agrees with cena because cena is singular:

• cena = price
• cena może... = the price may...

So cena gazu może... means the gas price may...

This is a very useful question, because iść and pójść are related but not identical.

• iść = to go, to be going
• pójść = to go, but more like to set off / to go as a complete event

In this sentence, pójść w górę is an idiomatic expression meaning:

• to go up
• to rise
• to increase

So here pójść is not really about literal walking or movement. It is part of a fixed phrase.

You will often hear:

• ceny poszły w górę = prices went up
• koszty mogą pójść w górę = costs may go up

Literally, it means:

• to go upward

Naturally, in English, it means:

• to go up
• to rise
• to increase

It is commonly used for things like:

• prices
• costs
• temperatures
• rates

Examples:

• Ceny poszły w górę. = Prices went up.
• Inflacja może pójść w górę. = Inflation may go up.

So in your sentence, it is a normal way to talk about an increase.

Because w górę is a set expression meaning upward / up.

• góra = mountain / top / upper area
• w górę literally suggests movement upward

In this phrase, it works adverbially, like English up in go up.

So:

• pójść w górę = to go up
• iść w górę = to be going up
• windować coś w górę is different and much less neutral

For a learner, it is best to remember pójść w górę as a chunk.

Because Polish often uses the present form of a modal verb together with a future time expression to talk about the future.

Here:

• W przyszłym roku already tells us the time is future.
• może does not have to be changed into a separate future form here.

So the sentence naturally means:

• Next year, the gas price may go up.

This is normal in Polish. The future meaning comes from the whole sentence, especially the time phrase.

Yes. Może wzrosnąć is a very natural alternative.

Compare:

• cena gazu może pójść w górę
• cena gazu może wzrosnąć

Both mean roughly:

• the price of gas may rise

The difference is mostly stylistic:

• pójść w górę is a bit more idiomatic and everyday
• wzrosnąć is slightly more formal or neutral

Both are correct and common.

Not necessarily in a literal sense. Polish often uses the singular when talking about the general price level of something.

So:

• cena gazu = the price of gas, as a general concept
• ceny gazu = prices of gas, if you are talking about multiple prices, tariffs, suppliers, or categories

In news-style or general statements, the singular is very natural:

• Cena gazu rośnie. = The price of gas is rising.

Yes, Polish word order is fairly flexible, although some orders sound more neutral than others.

The original sentence:

• W przyszłym roku cena gazu może pójść w górę.

This is very natural and neutral.

You could also hear:

• Cena gazu może w przyszłym roku pójść w górę.
• Może w przyszłym roku cena gazu pójdzie w górę.
  This changes the structure and nuance a bit.

The original version is a good standard model because it starts with the time phrase and then gives the subject and prediction clearly.

It is mostly neutral, with a slightly everyday feel because of pójść w górę.

• W przyszłym roku = neutral
• cena gazu = neutral
• może pójść w górę = common, natural, slightly idiomatic

So this kind of sentence would sound fine in:

• conversation
• news
• everyday discussion
• articles about prices or the economy

It is not slang, and it is not overly formal either.

A very useful pattern is:

[time expression] + [noun phrase] + może + [infinitive phrase]

For example:

• W przyszłym roku cena gazu może pójść w górę.
• Latem temperatura może wzrosnąć.
• W przyszłym miesiącu koszty mogą się zwiększyć.

And inside the sentence, remember these smaller patterns too:

• w + locative for in a time period
  
  • w przyszłym roku
• noun + genitive for of
  
  • cena gazu
• pójść w górę as a chunk meaning go up / rise