Good question
Balanced team has/needs:
1) balance between quallity and variety of players and tactics
- players for every possible position (there is 14 possible positions) which enable manager to use wider variety of formations
- strong substitutes and reserves (strong balanced teams have 2 starting 11 squads)
2) balance between players value, wage and skill
- player who has overall 3-4* doesn't have to be bad/weak; if he is in good form and his skill points were logically added, then he could be useful (GK and defenders who have low attack skills or attackers with low defensive skills)
- long-term players (who can play on high level for 3+ seasons) save money and tokens

This is my team now, it is not fully balanced because i haven't finished with transfer activity this season, i will change few players, then it will be fully balanced...
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