When a sentence ends with '来了', is it the aspectual or modal particle 了?
“来” is an action. So it is aspectual. It indicates the action has ended.

“来” is not a situation. Although a situation that "I am here" can be inferred from the fact that the action of coming (来) has ended. So it is not an indication of new situation.

You can say "我在了", in which the "了" is modal. "在" here indicates a (new) situation.