How about this?
A limit on the "mana" you can earn from games daily. Such as enough to buy a common, but no limit from the "mana" you earn from raising dragons.
Dragons you raise up from egg to hatchling would net you 500 "Mana"
Dragons you raise up from hatchling to adult would net you 500 "Mana"(this excludes frozen hatchlings)*
This would include all dragons bred or caveborn.
Since you don't have to raise the hatchling to adulthood to get points, people who don't like the dragon won't be hurt by raising the dragons to hatchling and can instead toss them to the AP where they are likely to be picked up. Getting points from bred eggs means people working on projects can continue without having to have egg slots used for eggs they don't want/need.
I would hope this would give more reason for people to pick up commons because raising them would bring them closer to getting a rare dragon.
I think the prices should be as so.
1000 "mana" for a common(You can earn this daily from games)
10,000 "mana" for a uncommon. 10 adults/20 hatchlings raised. 10 days gaming.
100,000 "mana" for a rare(Could included CB Holidays durning their season)*2. 100 adults/200 hatchlings raised. 100 days gaming.
1,000,000 "mana" for a prize dragon and/or dragons not availible in cave*3(Could include holidays not durning their season)*2. 1000 adults/2000 hatchlings. 1000 days gaming.
I would hope it would give people more reason to raise common as they are easy to get and wouldn't "waste" a slot anymore. This might help keep rares rare, and let people not fast enough/not have a lot of time still get them. This also means new players can get a silver/gold before they even have their silver trophy. The game part of it would allow people who don't want to collect more dragons be still able to get golds for scroll completion.
Yes it would set standards for what is rare/uncommon/common, but that is already in the trading market. How many people do you see offering golds for Trios? Sadly I doubt you can even offer Silvers for golds anymore. The only things I usually see when I look is people wanting to trade their golds for 2gen tinsels/shimmers or many hatchlings of the new release.
*excludes frozen hatchlings as they do not take the same time as adults take to raise.
*2 Holidays would still be limited to 2 CBs of all but Halloween
*3 Limited to one of each dragon a year. GONs would be limited to one overall.