In my opinion there are few sources of coins and generally the only ways to get them: free coins from shop, donating and daily quests. So i think you should add some more. For example, you can add GeoPass(like BP in othern games), where you can either pay some additional money each season or you dont pay, but still get some rewards. So you get some points from ranked and advance every X points. You can also add more decorations, so people who dont pay can get not only coins, but those decorations and you can make some season-exclusive rewards for those who pay.