Deck # Limit
I'm sure this has been brought up numerous times before, but I'll add what I think are some new thoughts here.
When the game launched, there were 294 cards, 232 of which were mechanically unique (for example, A1 153 "Marowak ex" and A1 264 "Marowak ex" are simply cosmetically different, not mechanically different - in terms of deck building, they're the same card).
Now, as of March 18, 2025, in the Triumphant Light set with PROMO-A cards released up through P-A 049 "Snorlax", there are 724 cards, 540 of which are mechanically unique. There are now about 2.46 times as many cards as there were at launch and 2.33 times as many mechanically unique cards as there were at launch. In theory, if the max deck capacity of 15 decks saved per accounts were to simply scale linearly with this, there should be either 37 or 35 deck slots by this point (2.46 * 15 or 2.33 * 15, respectively).
And I don't think it should scale linearly. Each set of new cards changes how old cards can play with new interactions. Genetic Apex's Magneton card got new life when Space-Time Smackdown's Magnezone's card released - each set of new cards adds both entirely new deck-building options and changes up old deck-building options while old decks can still function. The introduction of Leafeon ex in Triumphant Light has changed Celebi ex decks, but Celebi ex decks made pre-Triumphant Light can still function, for example. It might be worth keeping one or more of each, for some players. Accordingly, I think it should scale more than linearly. If I had to ballpark a number, I think we should be able to have at least 50 decks saved at this point, and that could still be a good enough number when the next set drops at the end of next month.
Otherwise, the conundrum myself and many other players face is that it's not even possible to save a copy of just 1 version of each major deck archetype being played at any one time, much less any variants. If you want to also save some less viable but fun decks, you're going to have to delete some of your good ones. If it were just about winning, then you'd only need to save maybe 2-3 decks, but playing 2-3 decks gets old quickly. It's fun to mix things up and be viable, and it's fun to mix things up while not being as viable. The current limit completely hamstrings the player's ability to do this and creatively play the game. Battles would almost certainly feature more deck diversity if we could have more decks on our account - I know the decks I play online would vary a lot more if I could fit more, at the very least. It's also a completely arbitrary limit set when the number of cards was under half what it is now - it simply feels bad from a UX perspective, and to be honest, 15 was pushing too few even at launch. It's beyond time to increase the limit.