If you maybe remember the pokemon games had a daycare. When you put the pokemon there he/she got XP and lvls. It costed some money, but you could lvl a few pets that were anoying too lvl that way.
I once even forgot a pet in daycare and i had too pay some sick price .
So i was thinking if it would be a great idea too implent this. Limit 3 pets max.
Prices could go up like this:
Day1 5g lvl 1-2
Day2 10g lvl 3-4
Day3 15g
Day4 100g
Day5 150g
Day6 250g
Day7 350g
Day8 550g
Day9 700g
Day10 850g
Day11 1000g
Day12 1250g
Day13 1500g
Day14 1750g lvl 20-23
Day15 2000g lvl 23-25
And your pet would get 1 lvl a day---- 3 days for 1 lvl after lvl 20. It would also be great moneysink .