I think about this a lot when playing fire emblem, the age old issue of a strong unit that kills incoming enemies to the extent that it winds up dead, frequently.
This would change a lot of the game’s mechanics, but it would be interesting to have some agency over how your units react to being attacked. Maybe this exists in newer games.
The only way I can imagine implementing this in GBA would be allowing you to smash B when under attack to prevent your unit from striking back. Ideally would simply be an option you choose in battle, whether to strike back once, twice, or not at all. Or even something you can set at the end of your turn for units that have finished their move. A simple “Defend” in addition to the “Wait” command.
*(Sort of a tangent but this is the direction I had hoped the series would take, not complicating the units themselves or focusing on their customization, but deepening the tactics available to you. *
Also had an idea about a Support system in which two units that act consecutively, you have the option of choosing one of two outcomes after seeing them both. As in, both units attack an enemy, you get to choose between unit A going first, or unit B going first, after committing and seeing both outcomes)