SMT+Fire emblem has me so excited you have noooooo idea oh my god. Also Yoshi's Epic Yarn or whatever it is is looking pretty amazing too. Kirby's epic yarn wasn't hard but really what kirby game is? They're still great games.
Also description of SMT for the uneducated, basically most of them revolve around demons invading the world in some manner and humans fighting back against them. Seems fairly generic until you realize that one, there is a morality system (law, neutral, chaos) that is typically fairly well executed, and two demons are treated similarly to pokemon in that you "negotiate" with them, (you litterally have a conversation with them, it's actually pretty neat) and get them to join you. You can fuse them to make new demons, and unlike pokemon you are expected to change your demons fairly regularly as they level fairly slowly.
The games are also punishingly difficult, well known for throwing around instant kills nigh constantly later in the games, and also well known for being maybe one of the only JRPGs where buffs are actually worth the MP used to cast them, and in fact are quite consistantly very useful on bossfights and tough enemies. Persona is a part of the SMT series, but it's a spinoff, and in my opinion recently has ventured far enough from SMT to stand on it's own legs. No one's really sure if Pesona'll show up in the FE+SMT game. I love persona, but I really don't think it'd mesh very well with Fire Emblem.
tl;dr: SMT is a pokemon-esque game with crushing difficulty and actually good instakill/debuff/buff spells.
Now to inspire a touch of fear in any Fire Emblem vets, instakill spells+fire emblem permadeath. (once more nobody's sure if permadeath spells will make the transition to fire emblem unscathed but man if they do)