I've played many titles of fire emblem. :) Here is a list of things that I've made(that are my opinion of course) to improve future titles. A majority is based off of things in fire emblem fates since that is the most recent title a future fire emblem game would most likely derive content from this version.
-When using warp you automatically "wait" after teleporting. It's not even an option since this could activate a personal skill such as Kiragi's personal skill. Still provides little restriction to movement without overpowering opponents.
-skills such as aether, sol, luna, astra, etc. are unable to activate criticals.
-if aether/astra activates when attacking shield gauge only goes up for the first attack(prevents the individual from potentially gaining a full shield bar, etc. in 1 battle.
-the return of nihil(only allowed on 1 unit, perhaps endgame content only 1 rewarded per file and only 1 unit with this skill is allowed over wifi): negates skills on enemy unit in combat with this unit and the unit with the skill nihil, basically if you put nihil on a unit there's no point in putting other skills on it since they'd be useless including something like +5 hitpoints.
-When "replicate" is used the replica has no items in its inventory.
-miracle activates at 100% only if an enemy's lethality activates otherwise it has regular activation in all other conditions. (essentially these 2 negate each other, miracle's sole purpose to prevent lethality's bypassing of damage infliction to win).
-Cancel returns-ability to completely negate a move(boss only obviously but still annoying to face, dragonskin was underpowered compared to this).
-Regression-new ability that would cause dual guard shield bars to drain rather than increase when in combat with this unit.
-Immunity-new ability that would negate status effects on this unit
-there needs to be a weapon that targets each specific class such as "vowbreaker" bonus damage to priestess/monk, currently restricted to hammers, dragon-based weapons, and a bow that targets tome wielders
-weapons that grant bonus damage to all weapon types: currently missing shurkien, bow, lance, etc.
-new weapon that gives renewal ability(monk/priest restricted)
-Return of the Runesword(heals damage dealt), with other weapon equivalents such as a Rune lance, Rune bow, etc. (all placed with the same restrictions as Nosferatu)
-Buffer sword, buffer bow, buffer lance, buffer shuriken, buffer axe, etc. : weapon that grants the skill "nihil". The weapon must be wielded for it to take effect. I see this as "overpowered", so perhaps endgame you're given an option to choose only 1 weapon the bow, shuriken, lance, sword, etc.
-Return of Venin weapons for usage
-weapons that grant "breaker skills" return(from fire emblem awakening)
-guard stance change: if pair up users switch, shield bars drain completely.
If the enemy's attack misses or deals 0 damage the shield gauge is not drained when full
-attack stance change: nearby ally does regular damage but cannot follow up, critical or activate skills
-debuffs(not self-inflicted) such as those caused by shuriken, setsuna's yumi, drain defense, draconic curse, etc only take effect if damage was dealt.
-abilities such as poison strike, savage blow, etc. only take effect if damage was dealt
-self-inflicted status caused by things such as dragonstone(+), silver yumi, etc. only lower stats when the user initiates battle
-freeze-double accuracy halve usage
-rescue staff and hexing rod having the same range as freeze
-rescue staff: can only be used on a unit that has already moved, is within range of an enemy unit, and can no longer move hence the name "rescue"
-ward staff(returns): doubles a unit's resistance(wears off over time); -20% every turn until normal in 5 turns
-critical ward staff(new): protects an ally from criticals wears off after several turns.
-poison: reintroduced via reintroduced venin weapons from past games with standard effect
-sleep staff: reintroduced via sleep staff from past games
-paralysis: reintroduced via new weapon/skill exclusive to character such as MU or late game unit or caused on maps by pitfalls, by enemies, etc.
-restore(staff): reintroduced to heal status
-shuriken users such as dread fighter/master ninjas deal half damage from a distance and deal regular damage up close when using shuriken
- bow users such as snipers/ballistician/bow knight utilizing the skill point blank deal half damage up close, and regular damage from afar.
-Once you have beaten the game(post game)weapons provided to you via allies in MyCastle now are from +1(most common) to +7(rarest)
-You have the option to decide whether or not you want your characters to be "recruited/purchased" that have no shield if your team loses, etc.
-You are only allowed to acquire skills through actual battles being completed with enemy units using weapons equipped(game won't allow you place units such as singer Azura with a bow in the inventory if she has no wieldable weapon equipped.
-The throne is the only place where you can restrict movement to your unit.
-You have the option to decide whether or not to allow your team to be "taken" to an enemy's castle if they visit your MyCastle.
-You have the option to decide whether to allow enemy units with DLC skills/units to combat your MyCastle.
-losing or forfeiting a battle grants 0 battle points.
-Only one bond unit is produced for each card(1 per person you "visit") with multiple options appearing where you select the one you want.
-experience DLC should be easy for any character to level up(a lvl 1 archer can gain a lot of experience by killing a level 1 monster rather than being forced to wield a "blessed weapon" that deals 1 damage thereby making it nearly impossible for them to get a kill at their current level.
-experience DLC monsters run only not retaliate with ridiculous abilities that restrict movement(defeats the purpose of the DLC by slowing you down), etc.
-DLC like museum melee would grant higher level weapons such as +3 - +7 very often compared to getting them in MyCastle by reward from NPCs.
-the ability to capture any enemy boss unit(aside from plot restricted ones such as endgame boss, etc.) to use on your team. These units should have their own critical quotes, images in battle, unique stat growths, etc.
-if future remakes are made of past titles such as sacred stones there should be no massive changes such as "a customizable unit", excessive relationship focus with children, etc. Just stick to the original game primarily with bonus content such as MyCastle returning.
-allow DLC music to be played in MyCastle and added to options under change music. I'm looking at you Vanguard Dawn...
-playing on classic mode provides x 2 reward chance post game. For example, you gain double battle/visit points from MyCastle battles when people battle/visit you or vise versa. You recieve an increased chance for rarer items +3 - +7 weapons etc. from NPCs in MyCastle. Units obtain double experience from battles(unless difficulty is changed or the option is changed by the player, option appears before battles).
-ability to save several wifi matches(like in the recent pokemon games, super smash, etc.)
-And please bring back the following recruitment theme...: D