My suggestion

CCW1208

I am an old player and left MPQ for 6 months ( I have experienced Thorbit and Gritty era). Now I played other app game. 
The balance of character is major problem of MPQ. 

1. old 5* have less hp and match powers than new 5*
2. 4* can not get enough match power to 5* even when 4* boost to LV 450

The power can be inflated to different level. That is not bad idea. Because you can create other characters to counter meta team.
 
The general stats( hp and match powers) should not be inflated. Because it will make your old character useless and decrease variability of game. This also make most 4* covers became meh

I am not here to ask something. But many of my MPQ friends leave MPQ gradually. Most of them are day 1200+

That is my suggestion 
1. All the 5* should have same level of hps and match power (Its easy part)
2. Like other games, all the 4* can transformed to 5 * by some game mechanics ( 50 4* covers to 1 5* cover). then keep 5* new rule. 

Maybe the Dev won't take care of this.