Remember that evasion can be very stupid late game if you got a lot of health and damage reduction.
If you are level 100 with a ton of stats against a level 1, it will deal near insignificant damage to you (HP regen can counter it even if that is enabled). Adding an evasion system multiplys your effectiveness menaning you not only take proportionatly less damage out of total HP, but also reduce more damage via armor and evade most attacks. This is a product of 3 factors which gives and unnescescary uberness against early game badies at high levels which ultimatly can be looked as a design flaw outside of linear games where you will always fight badies with increasing stats to match.
So what do I advise? Well use evasion sparingly. A huge monster like a gient granite golem or wendigo are hardly going to be able to evade atacks, even if they come at a snail rate towards them. Small monsters like a murlock or killer rabbit (lol) would be hard to hit with slow attacks as they could easilly move out the way.
Thus you could have a crule but unique agility cutoff system. Monsters than can evade require a minimum amount of agility to even be able to hit, a stright line cutoff to hit them that drops from a evasion strength down to near 0 (5% probably is a good amount if you think 0 is too cruel). Any Agility after the required will steadilly bring up the evasion chance until a maximum is reached (after which agility no longer increases hit chance). The better the evasion magnitude, the higher the requirements to be able to hit (the cuttoff) and the lower the maximum cutoff.
Examples....
No evasion (eg a building), always 100% chance of hitting.
Light evasion (eg a footman), capped at 100% with cutoff at 80%.
Medim evasion (eg a bandit), capped at 95% with cutoff at 60%.
High evasion (eg a ninja), capped at 90% with cutoff at 40%.
The cutoff is a product of agility againt level. For example against medium it could be twice agility has to be greater than the enemy level while on high it must be a tenths of your agility must be higher than the enemy level. The amount of agility past the cutoff to hit maximum could be the same between all evasion types (like 100 agility).
The advantage of such a system is it creates an annoying (in a good way) bady neiche. The high evasion badies can only be hit by heroes with insane amount of agility and as such you need one to do well. Low agility heroes like a strength hero could have high physical damage yet would never be able to kill high evasion badies. The low and medium evasions classify the amount of agility one needs to actually be able to fight well while the high requirement to max hit chance means that strength related heroes will still get their damage reduced by continious misses.
The only flaw with this system is that you must be careful to greatly weakon agility heroes so they need strength support (like low health or less damage) otherwise you will fall into the trap of agility heroes being too strong.
