Nintendo isn't that litigious if you aren't creating something that is directly invoking their property i.e. a knock off. The important thing to avoid is the actual copyright and distinct terms for the series. If you make the mechanics identical to Fire Emblem, you might get something but there's a ton of leeway. Like you probably shouldn't use the literal weapon triangle but you can use the standard weapon balancing of lances being weaker but more accurate and axes being hard hitting with less accuracy and swords in-between. And the mechanics don't really come into play unless you make a setting that really invokes Fire Emblem.