Many RPGs center around combat, so it's important to get it right. So I think my combat is going to blend ideas from Pathfinder 2e, D&D 5e, and Warhammer: Age of Sigmar Soulbound.
The main idea I'm borrowing from Pathfinder is the idea of actions. Every player gets three actions. Everything they can do uses those actions. Some things--movement, attacks, a few non-attack spells--take one action. Other things, like loading a crossbow, or casting most spells, can take two or three actions. Some things--for example, certain powerful spells--could take up to 6 actions, and have to be split between turns.
In addition, players get a reaction each round. The reaction works for opportunity attacks, and other things that only take a single action. This way you can ready an action--spend an action on your turn to set the conditions to trigger an action.
Pathfinder 2e uses penalties to make it difficult to make multiple attacks on your turn. You can use two actions to make two attacks, but the second attack has a -5 penalty. If you use a third action to make a third attack, you take a -10 penalty. I don't really care for this, and would probably use something like D&D 5e's Extra Attack feature. You can only make one attack during your turn unless you have an ability that lets you make an extra attack. And that includes spell attacks.