My Suggestions

I have improved my suggestions, so I'll post them here again.... There is still scope to improve the game and get the balance right....
Gym Daily Challenges..
1) Spinning a gym - with a Mon in the gym ....
Gym Badge - Bronze 10 stardust, Silver 25 stardust, Gold 100 stardust
2) Training (reinstated) Max 10 coins (5 victories)
2 coins per win - first 5 count for coins and stardust.
Gym Badge - Bronze 5 stardust, Silver 10 stardust, Gold 50 stardust
3) Defeating a Mon in a rival gym (first victory)
10 Coins - 100 stardust.
4) Claiming a rival gym (first victory)
30 Coins - 200 stardust....

The aim is for gym daily challenges to total 50 coins...
The 50 coin per day limit for Mons returning stays.
Gym daily challenge stardust - max possible - 550

As suggested by others Mon in gyms also earn stardust up to a cap of 500 (when returned)...
1) Training victory - 10 stardust. 1 berry
2) Defending victory - 50 stardust.
The cap is per Mon so if 2 are returned in the same day you can get 50 coins and 1000 stardust.

The aim of the above is to make gold gym badges attractive, that will drive player competition.

Daily gym challenges also mean fairer rewards, and more activity.

In addition the game should be recalibrated up to level 100 and players at level 40 assigned a level of 40-60 based on their XP.

This is to allow players to power up super 3000cp + mons for use in raid battles .... there can be higher level raid battles designed for elite players perhaps designated solo battles.

These 3000 cp+ Mons should lose motivation very quickly in Gyms - there should be some additional loading based on hours in the gym.

As well when placed in a gym any mon > level 40 performs as a level 40 Mon in combat, but loses motivation faster.

I'm not saying Niantic can make these changes quickly, just that they need to reassess and plot the future direction,