What is a good antonym for "commiserate"?

The Cambridge Dictionary defines Commiserate:

to feel or express sympathy for someone’s suffering or unhappiness

In my experience/opinion, it's more than sympathy and implies a joint suffering and is often used ironically. For instance, a sports fanatic might commiserate a team's loss with a fellow fan.

Using a similar sports example, what might I do with a fellow fan if our team wins?

Since you liked wallow in your victory so much, but prefer a single word, I will propose

revel

verb: 1. To take great pleasure or delight: She reveled in her unaccustomed leisure.

2. To engage in uproarious festivities; make merry.

Related:

noun: (often revels) A boisterous festivity or celebration; merrymaking.

(thefreedictionary.com)

I think this is the word you are looking for, because I have heard news reports that after a particular game, some revelers have gotten out of hand. This has given me the impression that reveling after an intense sports match can be an intense group experience.

If two fans aren't commiserating, they must be celebrating

to commemorate (an event) with ceremonies or festivities: to celebrate the success of a new play.

This is definitely appropriate, as this snippet from an Independent article on the Chicago Cubs winning the World Series:

Chicago's landmark victory in the World Series has triggered a wave of emotional and touching videos of elderly fans celebrating back at home

Congratulate

Congratulations tend to pass from the loser (or their fans) to the winner (or then fans) and so are a direct opposite of commiserations, which are from a winner to a loser.

to express pleasure to (a person), as on a happy occasion: They congratulated him on his win.

(Dictionary.com)

This is often seen at the end of a game show in the form of:

Commiserations to [losing team], but congratulations to tonight's winners [winning team]!

This also fits if you want to fit this into your situation of two winning fans:

They congratulated each other on another win for their team.

Russian fans congratulate each other in the street after the finish of the Russia-Czech republic soccer match at Euro 2012, in Moscow

(Alamy Photograph Title)

One possibility, especially in the context of others' suffering, is to be/stand aloof:

distant, unsympathetic, or supercilious in manner, attitude, or feeling

Rejoice

Rejoice, which means: "to feel joy or great delight," is another celebratory word with less potentially negative connotation than revel.

Examples of rejoice in a sentence:

We all rejoiced over our friend's good luck.
We rejoiced over our unexpected victory on the soccer field.

(See Merriam-Webster)