Head to Head battle

Think of mechanics. As a couple of gears connected.
1 leads to another.

The gears need to go smoothly. No weird shapes and all.

Here is a mechanic for combat resolution:
- Check statistics table for number of dice to roll.
- Roll the number of dice.
- Compare results with the statistics table.
- Remove or change pieces on the board.

Sometimes, one wants to make a mechanic simpler:
- By removing the table.
- Lowering the number of dice. Even 0 is an option?
- less pieces on the board means less removing, placing or changing.
- Changing pieces could be removed from the mechanics.

The specialists here can tell you much, much, more about how to look at mechanics. I am only a simple amateur that stopped playing board games.

it can be hard to not have your game grow to a behemoth behind your back (i find it particularly hard to limit the number of cards in a deck).
i can appreciate the desire to have your next x expansions all planned and ready to print, as soon as the base game hits the shelf. its a trap i, and many others have fallen into.
for some designers this is a good idea. maybe the expansion is sold separately to keep the base game stream lined and/or cheaper for new customers. maybe the expansions are a continuation of the story. maybe tne expansion is "good but not essential stuff". or more often " if we sell it in 2 boxes we can make twice the cash"
the problem is that you cant sell an expansion for a game that no one has bought.
it is better, if you can, to make your base game, its mechanics and terminology flexible enough to make expansions possible. t future proof it.
as a random example, adding that extra stat.
if the stat isn't used (or is rarely used) in the base game it shouldn't be there.
lets say its movement speed, but your other mechanics mean its largely irrelevant. instead of making that stat, future expansions that would need a speed score use the more flexible, "power" stat that was there from the start (maybe for combat).
or add "speed" but use it more in the base game ( say for movement) and in an expansion as an additional use (now some cards have an initiative order, and speedier players,play first)
you can add speed as a new stat entirely but it shouldn't appear on any new versions of an old card type. ie if none of your characters had base speed, the new characters shouldn't either. instead they can now obtain "boots" that give them the new stat.