Neither game was particularly balanced.
Both favored survivor in casual and competitive play.
If anything, the other side will start off strong because people won't know how to play yet. Then through gradual neutering and people figuring out how to exploit the mechanics. The other side will become too strong and the devs will let it go because they don't want to mess with the code anymore.
Then pubs will just be the usual nonsense as it is in any game without a penalty system. So with any type of resistance equals auto ragequit and every game will once again become a 1 map, hell sometimes 1 half affair.
If Turtle Rock decided to help Valve with a Left 4 Dead 3. They should go for the rumored RPG element rather than more of the same. Similar to Resident Evil Outbreak. Its the only way to make the game similar, but different. They would also save themselves in the long run from the constant whining about survivors by just including the L4D1/L4D2 set along with maybe 4 newbies or 1 new one to replace Bill.
Give them each something worthwhile...
Louis - Better spread on assault rifles
Zoey - Better spread on the hunting rifle and double pistols
Francis - Better spread on autoshotties
Coach - Can tank more hits from infected
Ellis - Better spread on T2 snipers
Nick - Better spread on T1 weapons and magnum
Rochelle - Takes more damage, but is faster than all the other survivors in every scenario
Random new character - Heals themselves and others to 100 percent every time with a medkit. Everytime they use pills or give pills to others, it offers 50 permanent HP instead of temporary, but the offset to this is a slower character
That's all pretty basic cvar type stuff, but the idea is there.