Something that I'd love for Fire Emblem Heroes to implement is a new item that can be used as a universal merge +1 for all units and/or based on their color.
For example, in Alchemist Code, you use character shards that you can from duplicates to raise their level cap (thus their stats), much like Fire Emblem Heroes. But you can also use Rainbow Shards that can substitute for any character's shards, and there are also shards based on each element, so a Fire type unit can use Fire Shards to increase their max level.
The same thing also exists in DanMachi Memoria Freese, duplicate units become bonds that can raise a unit's level cap and status board up four levels after 60, but with some time and effort you can collect materials that can be traded in for a bond that can be used for any adventurer or assist depending on which kind you get.
Right now the only way to strengthen your hero in FEH is to have another of that unit. Granted in this game, merging provides a lot less of a buff than in the above two examples, but it'd still be nice to work for some sort of reward that can be used to boost our favorite units that are difficult to acquire multiples of (particularly seasonal units, but also units just locked at 5 stars).