No trainers of any kind on the first turn. Now for a more radical change:
Each turn, there is a limit to how much damage a player can deal with their attacks. If the damage they would deal exceeds this, it only does that much damage. In the case of damaging multiple Pokemon with an attack (Landorus, Darkrai) the damage to the Active is prioritized. If you're using Registeel and not targeting the active, you choose how to spread it. If a player is attacking with a Stage 1, add 20 to these numbers. If they are attacking with a Stage 2, add 40 to these numbers.
Turn 1: 20
Turn 2: 60
Turn 3: 100
Turn 4: 140
Turn 5: 180
Turn 6: 220
etc.
This prevents donking, and is also a slight nerf to many basics as they can easily exceeed the turn caps. This also hurts BKEX (a card I personally despise, 200 damage and a drawback nearly negated by an Energy Retrieval on top of a 180 hp basic? No thanks) as it will not be able to OHKO an EX prior to turn 5, and unable to KO one buffed by Eviolite/Giant Cape/Aspertia until turn 6. It doesn't hurt Stage 1/2 attacks for the most part as Garchomp can do 100 t2, Hydreigon can do 140 t3 (if you're attacking with Hydreigon turn 2, LIST WANT) etc.
Another change is to make Revive an Instant (think MtG) or Trap card (think Yugioh). This allows you to play a basic back from your discard pile on an opponent's turn. This doesn't really help EXes and other big basics, but its huge as it allows you to get back a weak Tynamo or something after they Catcher and then evolve it on your next turn.
Rather than a limit on EXes, I like this rule: Each basic with over 100 base hp loses 10 hp for each other basic with over 100 base hp that has been played before it by its player. If the basic is an EX, it instead takes 20 damage per basic with >100 hp played before it by its player. Rather than the other EX limiting rules that have been suggested, this does not make other big basics such as Terrakion incredibly op, and rather than adding or arguably even removing some strategy from deckbuilding, this adds more strategy to actual gameplay, something I find the Pokemon tcg lacking now.
Sadly these rules are far too complex for the Pokemon tcg to ever implement them, especially because the BW-on sets are all based on dumbing the game down for little kids in both rules and cards.