I'm watching my brother's football match tomorrow.
This is only odd in that “I’m watching” can mean that’s what I’m doing right now, so adding “tomorrow” pulls you up short. But it’s a typical answer to: What are you doing tomorrow?
I'm going to watch my brother's football match tomorrow.
This could not be misconstrued as meaning a present activity. But, strangely, it’s still potentially ambiguous, in that it could mean either what I intend to do is watch the match or I’ll be travelling to the venue to watch the match.
This just reinforces what we always say, that context is crucial.