I'd guess I'm only about halfway through Echoes, but I'd say that's already enough for me to confirm my feelings about this. I really do enjoy the game and think it's good sauce overall, but some of it's mechanics and changes I'd rather not see in another Fire Emblem game tbh. This is the latest FE game though, thus potentially the most telling of what the next game will be like, so here's just a quick list of what I really do not want.
-Linear Class Tress/Minimal Classes
Probably my biggest gripe. Aside from the few villagers you have no say in what classes your units are. There also just aren't many different classes in general. Less options is simply less fun and gives the game severely limited replayability compared to the previous games.
-No Pair Ups
I love love love this mechanic! Added many layers of strategy and options, and Fates version of it was just perfect, so not having it is sinful! Okay technically there is a very light version of it in Echoes, but nowhere near as in depth or impactful.
Enemy units keep respawning on the map and you have to get through them to get to the actual missions, highly discouraging me from backtracking. Enemys in dungeons also respawn, though atleast there it's possible to avoid them.
Speaking of grinding! They basically took the most tedious and repetitive parts of Conquest (Faceless spam) and made it into a unit. This thing is just blatant padding. Honestly can we not have monsters in general? They're just not very interesting to fight, though atleast Echoes has more variety with it's monters.
-No Weapon Triangle
It's safe to say this wont be a thing in FE 2018 but eh listing it here anyway. A nice layer of added depth that has no business not existing in a FE game!
-Bland Maps/Rushdown A.I.
Awakening is also very guilty of this admittedly, but I wasn't fond of it there either. There have been some maps where the enemys actually don't move unless you aggro them, but on the majority so far they all just yolo charge. After Conquest we know IS is capable of well thought out map designs and enemy placements/A.I. so it's fair that I expect better!
-Overall Bulkier Units
On the fence on this one, I like it but I also don't lel. Perhaps it's just cause I'm used to Awakening/Fates where most everything dies in 1-2 hits, but I do think it raises the stakes and makes every move you make really count. Can't help but play more lax when my units can get crit twice and still live.
After writing this, I realize that's basically everything about the game lawlz! Well I certainly wouldn't mind the full voice acting and the continuing trend of archers actually being good. Other than that though, I really hope Echoes remains as it's own unique little thing, hate me for it!