The main issue I see with bringing back invincibility frames is that weapons work the way they were intended to work without them. However, I see many feel as some weapons are too powerful, making mistakes is too punishable, and people die too fast. I don't believe bringing invincibility back is the best solution, because then it would just return to the way lockdown used to be, which was not good.
However, I'm sure most would not mind a universal 40 percent damage reduction to all weapons in lockdown only. What this will do is make it so you have to try harder to kill someone, since one combo/charge/whatever would not kill you in one go. I suspect even with this damage reduction Blitz may still be able to one shot with the charge, but it will be far more difficult as you'll have to land a lot of extra hits, which gives time for people to move out of the way. It will also encourage more teamwork, as killing a person will take longer. People will have to team up if they want to take down a target fast, meaning a lone person would be able to distract multiple members of a team if he/she was good.
Also I was considering maybe bombs should be an exception to this rule or take a lesser blow, maybe 20 percent instead. They are not large sources of damage and a decent player would not be easily hit by a bomb. This however, is a toss-up.