I don't really think single pick is necessary not only because we've been playing the game for years without it, and from a balance perspective having more than one of a certain character on a team isn't really game-breaking.
Yeah, 2 or more raiders or cents or hitos on the other team can be tough. Not impossible, though.
The fact of the matter is some heroes are simply stronger than others. A 4 stack of Orochi's would not be as powerful as a 4 stack of Cents. It all has to do with pressure, and some characters have a lot more pressure in 4v4's. Unblockable pressure, 50/50's, etc. You also need to read your opponents in 4's believe it or not. Some Raider(s) might always let a zone fly in a group. Be prepared to parry it, and if he's not locked on, dodge it appropriately. If a Cent keeps doing a full punch in a gank, try and roll away or be more patient and punish it. Shugokis and hitos can be tricky since they have HA and can target swap when they charge their heavies. Either back step, roll away, or interrupt with a bash.
Just little things. Some players might try and be big brain gamers and, you guessed it, NOT let unblockables or 50/50's go. 
I've been in many games where I'll have been external blocking a Glad, and he got into his UB mixup. He feinted to a GB, and I externally counter GB'd. Same with Berseker. Cents have also feinted their UB's or punches to GB's as I was externally blocking them. The fact of the matter is to just keep playing 4v4 game modes. Yeah you'll run into 3 or 4 stacks of cents or hitos or raiders or whatever. Those characters are naturally good in 4's because of all they're wide arching AOE attacks and HA, and can hit really hard. Cent just has a lot of bash UB pressure. It also depends on how good your team mates are, and that can vary greatly if you play with randoms. I've had games where my team and I would destroy 4 stacks of Shugokis. Then I'd run into another 4 stack of Shugokis later and get absolutely destroyed. It happens.
I don't think it's as unhealthy and unbalanced as you say it is. Just challenging.
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