Actually, that's brilliant xd I never even considered a 30 card format. The maximum 2 cards of anything would really shake up things. I'm still worried about the issue of the queues spreading up too much, but I'd like to try out a 30 card format.
I have another idea that I'd like to add, although I'm not certain of its viability. One thing I saw in a different game (namely, LoL) is Rotating Game Mods.
While I never thought about making the fourth official game mode one that changes; sounds like a good idea. There are actually three semi-official Formats that used to be in the Play! Pokémon Rules and Formats document... but the other two (2-on-2 and Team Battle) would most likely require too much effort to add to the game. 30-card may be a problem as well, depending on how the game is programmed. That is the real restriction, I think; a bonus like this needs to be easy for the Dev Team to implement; I get the feeling they are operating on a shoestring budget as is, or else just constantly burdened by the sheer volume of cards and mechanics already involved in this TCG.
Making the fourth mode a variable one, maybe that changes with the Reward Ladder might be best. That brings up a second concern; the mode has to work well with the Reward Ladder. While I might still try these if they didn't contribute to the Ladder, I fear most would be forced to choose or just automatically go with the three regular options because of the Reward Ladder. So, if some of these aren't too well balanced, the Dev Team might be skittish about allowing the same Rewards as everything else. Though, I am hoping the "rotating mode" might be okay; if they think a mode is "too easy", they can just make a less impressive Reward Ladder. Alternatively, just give double the Daily Rewards and no Reward Ladder; I think, so long as there is something, people would still give the variable mode a try.
If all the Dev Team can do is allow or disallow entire expansions, still call it the Legacy Format but make it clear that it rotates between past Standard Formats, plus the HeartGol/SoulSilver through Legendary Collection one they invented, which is currently the actual Legacy Format. Maybe even the PTCGO-Unlimited Format as well.
If they can handle it, 30-Card would be nice. Maybe a "No Rares" Format as well. Even though it won't work, I think a "No Pokémon-EX/GX" Format, just to prove the point.
Now, the final sticking point, if it is possible and permitted, is some of these might need their own ban list. However, dealing with updates and glitches in the real game comes first, then fixing glitches for these alternate game modes, and finally worrying about banning anything (if it is needed at all). Players need to understand that the bonus mode is just that; an extra toy the Dev Team is letting us play with, so if it isn't working well, they'll take care of it when they can... which might not be until its next time or two in the rotation.
If you do not have Private Messages enabled, it really limits you in discussing the Pokémon TCG.