If there is to be any "solution" to gym stagnation, I would think it would have to come by real-life organization of events. Niantic, Nintendo, the Pokemon Company, or other gaming, electronic entertainment companies, or organizations, running country-based tournaments for teams to own the most gyms. The contributing members of the winning team then earn rewards based on how much they contributed to the victory.

So for instance, Walmart (or any other company really, I just chose a name that popped into my head) sponsors a tournament.

Winning Pokemon Team gets discounts on Walmart items. You must control majority of the gyms in the state of Florida (just another random name that popped into my head) to win. You must also have some contribution to holding gyms to win.

Each time a player knocks an opposing team out of a gym, they earn say... 100 Tournament points. For every Pokemon they leave defending a gym, 50 Tournament points each. For every minute the defending Pokemon stays up, they earn 2 Tournament points per minute... Something like that.

The winning team is the one who has most possession of the gyms for longest time period during the tournament period (therefore translating to more tournament points).

Team members are awarded based on contribution.

Top 10% of Tournament point contributors - Maybe 20% Discount on Walmart items for 2 months?

Those who rank above 50% - Maybe 10% discount?

Those who rank 50% or lower - 5% discount?

Numbers, company involved in organizing the tournament, and figures are all debatable and can vary between tournaments. Maybe Niantic themselves can run tournaments and offer discounts to their in-game micro-transactions for the winning team.

For this to work though, they really need to sort out their cheater and spoofer problems.