One of the great things about Fire Emblem is that you cannot make a foolproof plan, so you make plans that give you as high a probability of success as possible. Say your unit has 50hp, and you encounter a group of 5 units that will kill you if all 5 enemies hit, but they only have a 60% hit rate each. You then have a probability of death around 8%.
Alternatively, you could play more defensively, and position yourself so only 1 enemy can reach you, but this increases the likelihood you'll be overrun by other enemies coming from somewhere else on the map. You don't know exactly what's going to happen there, so you have to decide whether the 8% risk is worth it.
If you remove all the probabilties, the game becomes deterministic, and you lose these tradeoffs. You'll know in advance - if I move Ike there, he'll have 4hp left. So the right thing is obviously to do that, then heal him up and then tackle the enemies coming from the other side.
So I wouldn't like getting rid of hit and crit rates. Skill activation being deterministic like in FE Heroes feels like an improvement, though. When you have too many probabilities in play at once, it becomes too muddy, and it ends up just feeling like luck. Both Radiant Dawn and Awakening occasionally had this problem. You have to take account of hit rates, crit rates, own skill activation rates, and enemy skill activation rates. It's simply too much to do a quick estimate.
Another thing I wouldn't mind them taking from FE Heroes is having smaller teams for parts of the game. You could be forced to split up your army, like Radiant Dawn part 4, but at a point where you don't yet have dozens of units, so each team is relatively small (4-6 people). Small teams on small maps works remarkably well in Heroes.
