What is the difference between "I'll play you next" and "I'll play with you next". Is there a grammatical error in both, it both means the same...
In modern English, they are two different verbs (or two different meanings of a verb, if you will).
The OED says
a. To take part or engage in (a game, sport, etc.); to participate in (a sporting match or contest).
Intransitive "play" has a range of meanings, including one very similar to 13a., but many other specific senses, only some of them competitive.