Its easy: create a new tier, let's say tier 11 so they only meet tier 10 & 11 tanks.
Move several OP tanks to tier 11 to balance mm, tanks like obj 430u, obj 277, objs… etc.
Buff the T110E5 to its original glory.
Reintroduce unnerfed version of Waffenträger audio E100 into tier 11.
Make gold ammo have less damage than standard ammo.
Add more tier 11 tanks such as super M60 for T110E5 players to grind towards.
Buff armour of super heavy tanks (introduce a new class… super heavies) since they severely lacked mobility. (since T95/fv4201 can move at 50 km/h with 350 turret armour… way more than what maus have, same for the Russian tanks.)
Alt gameplay mechanics are good however not balanced. ( eg. Arty, progetto, ebr, double barrels)
Suggestion is to stop introducing newer ones before they fix the meta.
Give players free gold every year to encourage them to keep on playing (eg. 10k gold per year spread over 12 months, requires you to be active 70% of the days of that month)
As a marketing expert, I feel that these changes would modernise the game.
Edit: To those whoever downvoted my comments, you guys are the reason why the community is so toxic, stop playing this game!