When you proliferate, you can choose any permanent that has a counter, including ones controlled by opponents
Adding a new card type is a pretty big deal, so it's almost easy to forget because there weren't that many standout cards, but it's going to be interesting to see how they handle battles in the future
Run as many events as you want during these periods (and plan how you'll distribute your two promo cards)
Whether you're a fan of Mew, the original Mythical Pokemon, or prefer newer options like Victini, there's something for you to find among the most valuable Mythical Pokemon TCG cards
Especially 2019 was a disaster with Pikachu releases, with a total of 233 cards
There are other grading services but these are the bigboys if you like