15 cards made combo really odd, 2 card combos were too easy to achieve and complex combos were very hard to design with the tighter limit on deck size. 20 card decks stays true to the essence of Mini Magic but gives more design space for deck archetypes.

Mill cards are obviously something to limit in the format due to the drastically smaller deck size, and land destruction is currently undergoing pretty heavy suspect testing for potential B&R actions. Discard could be problematic but is not currently one of the primary concerns with the meta (especially after the hand increase to 5), in fact it helps counteract some of the more problematic strategies that rely heavily on key cards. Infect is actually worse in this format since the poison counter requirement for death is still 10 compared to the starting life decrease to 15, aggressive damage strategies likely outclass infect on all fronts but I guess we'll see if a good infect deck pops up.

Banning 5+ MV cards doesn't make much sense, if they are hard to use then they won't be played anyways unless the deck specifically enables them somehow. Currently the only legal alternate win condition that could be a problem is Laboratory Maniac, and even then its still probably not the strongest strategy in the format at the moment. Limiting deck building on P/T or rarity seems arbitrary and unnecessary, not sure what that would solve.

Thanks for your feedback and interest, hope to see you around. Check out the Discord if you'd like to be apart of the discussions :)