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...
I feel that "I'll play you" is competitive, where as "I'll play with you" could be collaborative. So you could play someone at tennis, but you could play with someone in a doubles game.
I'll play you next means that you will be my next opponent. I'll play with you next might also mean that, but generally it would suggest some less serious kind of ludic activity.
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.
Dunno if it's just me, but while a child saying I'll play with you might sound innocent, an adult saying it sounds rather sexual.