I'd love to create a Smash Bros X Fire Emblem game where the entire roster is made up of Fire Emblem characters.
Characters
Instead of different characters, there would be different classes (Fighter, Pirate, Myrmidon, Assassin, Pegasus Knight, Mage, Druid, Dread Fighter, etc.) and each "skin" would be a different character from the Fire Emblem franchise. So if you chose a Swordmaster Class for example, it could be Navarre, Joshua, Guy, Lon'qu, and so on (as long as they are a myrmidon or swordmaster canonically). Just like in the Fire Emblem series, each class would have their own unique traits. Pegasus knights, Wyvern riders, and other fliers would be multi-jump units (like Pit, Kirby, Jigglypuff and others), but be lighter because of that. Characters could wield a variety of weapons however. For example, a warrior could set up his inventory with an axe for power, and a bow for range.
Weapons & Items
Once a character was selected, each player would choose up to 5 items (maybe only 3, haven't completely thought it through) to go into battle with. Items would include weapons, tomes, staves, and other miscellaneous items seen throughout the games (such as a dracoshield for more defense, or an elixir to heal yourself). Like in Fire Emblem, a heavier weapon would deal more damage, but cause the fighter to be slower. Different weapons would have different traits. Marth's rapier would have a tipper like effect (where the tip does more knockback & damage), whereas the Killing Edge would randomly decide when to deal higher amounts of damage. Maybe a brave lance would leave an opponent in stun for follow-up options. The Devil Axe would be extraordinarily powerful, but deal damage to its user.
Stages
Stages would just be notable locations seen in the games. There would be terrain however, which could affect units. Water, forts, forested areas, pillars, stairs, and so on would all play their respective roles in the franchise. For those who don't know, standing in a forested area in Fire Emblem decreases the chance that your unit will be struck, so in this Smash rendition, being in a forested area may regenerate your shield faster. On the other hand, being in water or snow may slow down the unit, or cause more hitstun when hit, or landing lag when coming down.
I would love to see how people would utilize the massive amounts of customization in an already very strategic game. I would keep the game fast paced, and go back to the melee era where it wasn't as rewarding to play so defensively (always shielding and rolling...). I would want the game to be as technical as possible, so there would be a number of ATs implemented into the game.
That being said this is all completely theoretical, I have no intentions of creating it, I'm sure there are many holes in this idea, so please don't bash.