dthey DID do this with civ 6. 2 or 3 units can now be combined (almost the same as stacked, tbh it is an even better mechanic, shame it comes so late). also, you can now stack unit types, so for example one tile can hold a warrior, a worker and a missionary. or, one tile could hold a battleship, a watered unit and an admiral. or, one tile could hold a rocket infantry, a military engineer, a settler and an apostle (I think at least, maybe not both the engineer and settler).
or, if you have formed an army, one tile could hold 3 machine guns, a builder, a great general and a missionary.. I think