I know it's long and looks boring but please read it :). And sorry for the bad English.
Here are the things:
-Effect:

+Ok, there're four main effects right Fire, Shock, Freeze and Poison. But I think poison is a little bit useless at times, first others effects are good at every situation (except their resist monster but that doesn't count) but poison, they're just useful when there're Mender or Silkwings, and even that the poisonous time is like so short and it doesn't happen everytime. I think you should add something more advance for poison to be scared of. I just saw people use fire, shock, mostly freeze and even stun but i didn't see no one with a poison vaporizer or poison alcheimer.

+And about the 3 other optional effect: Stun, Curse and Sleep. Like poison, sleep are almost useless. Who want to make monster sleep then they're healed up themselves anyway? I think using freeze are better, whenever I need to throw a vial to get the capsule, i choose Sleep first, throwing away with no regret. AND I think there's 4 main effects but why are there 3 effects (I'm a perfectionist and being obsessed with balance). I suggest a new effect: "Possession" or "Own" or "Hypnosis" (sorry :( can't find any fancy words) that you might have a chance to own a monster, they will be possessed in some second and attack their own kind.

-Weapon:

+Sword:

~Let me tell you this, swords are boring. I mean their attack maybe different but they sometimes looks alike. And people use it too much, make the SK world imbalance. I suggest some new kind of swords which isn't sword, maybe axe (yes spiral knight have it but i just saw an 0-star axe while i'm in Rescue Camp).

~OR Whip which gives you low damage but it's really comfortable to use, you can move while attacking and whip's attack speed are really fast. Their charge attack is to whip a straght line forward you and through the monster, thay may have a chance to be possessed :D.

~Or a drill, it looks a lil bit like a gun but with a drill head, the normal attack is the straight ahead attack, making the monster are unable to move neither are you, but you can be attacked by other monster. The charge attack is drill a straght line forward you while running (the difference with whip is whip make you stand, while the drill make you run toward with a really fast speed).

~The third one is the light saber (lol), it's attacking is like normal sword, but the charge attack is boosting yourself, make you move faster and the light saber is stronger and faster (and maybe its attack may be fancier).

+Gun:

~ Well, i'm a gunslinger. So I have all kind of guns so I have many suggestions. The alcheimer, well, they looks really normal to other special gun, and maybe aren't appreciated like his elemental brother - pulsar (Well, except the shadowtech one.). The alcheimer have things like the ricochets to boost it, BUT the ricochet just bounce to the left randomly, like 10 ricochet, maybe only 3 hit something, 1 of them is the wall. At least they should bounce two ricochets (1 to the left, 1 to the right) than ricochet TWICE, because the ricochet hardly hit something to get to the second ricochet and if it does, the second ricochet hardly hit somethings too.

~ And I don't know if it's a glitch or not. But when I use a Autogun line to hit Wolvers, Devillites or Gremlin. Normal hit: they dodge. Charge Shot: They stand there and take all the shot (except Tier 3 Wolvers). I heard people say monster dodge autogun shots. Are they just apply to normal shot?

~ New gun: Laserizor. Well, it's a laser gun. It will shoot a laser line (like the machine you can find somewhere in some slime grass level). The laser gun have a normal shot like a magnus, but weaker and shot through all the monster in line (the bigger the star the wider the laser shot is). The charge shot: Your gun will make a 3-5 second laser shot, you can run and turn while the shot are still shooting, but after 3 second you will stand to cooldown your gun.

~ Another new gun: Bomb gun. Just like in Team Fortress. This is a gun which you can shoot bomb like the Bomber Gremlin. You can shoot 3-5 bomb and its power depend on the star. The charge shot is shooting a bomb straight, when it hits something it will make a big explosion after some second (or immediately)

~ New Bow: A bow which you can shoot rapidly without reloading, but the damage sucks. The charge shot is making an arrow rain, arrow all the floor.

+Bomb:

~ New Bomb: A target, a scarecrow bomb. Which you can put a scarecrow in the map, the monster will try to attack the scarecrow (it has health). The monster will attack you back when the scarecrow are dead. (Yes, this is a little bit imba, but it might have some disadvantage)

~ New bomb: A sentry gun bomb (again, like in Team Fortress). Where you put a gun that auto hit the enemies, but the gun is easy to destroy.

+New Class: Staff (Magician)

~ Well, in a party max is 4, why do we only have three classes (maybe there's alternative class but this is just a suggestion). We could have staff and wand weapons. They are basically supporting class, but can solo. Their normal attack is attacking (lol), but their charge attack is support something (not heal because i think this will make the game easier. One more thing the charge attack need really long time (5 sec perhap) and require walking (like gun)

~ Since every kind of weapon are different in attacking, i think the staff should differ too. Their normal attack will set the nearest target at where you click or face, attack them with something falling down from the sky with a 140% accurate, it might hit nearby enemy to (but the damage sucks fyi). Like an elemental lightning staff will hit the a lightning strike from the sky to the nearest click or face target. The charge attack of it is make all your member (include you), add and increase their shock damage (will make them have a shock effect on their attack).

~ Another kind of staff weapons line's charge attack could be summoning. Summon a rock golem which doesn't attack but is the shield for your team (the max amounts of golem depends on the party (the first one will run around you, the second to the nearest member, and so on). Or 5 tiny royal slime. Or spirit, they doesn't attack and can't be attacked, they just stand in the map and make an effect to a random enemy.

-Monster:

+Gremlin: In my opinion gremlins are weak (except the mender they're dangerous and annoying). They're annoying yes, but are easy to kill, especially the fire ones.

~My suggestion is a new kind of gremlin, they're on a flying machines which have a drill head point to the ground, you can't attack them on air and neither are they. But they will fly around and when they're above you, they will star drilling down and hit the ground. At that time you are able to attack them. They take construct damage because of the machine, but when you destroy it the little gremlin came down and become a mechanic gremlin.

~Mechanic gremlin: They just run around, building load of things, like some sentry guns or some rocket fire, they need time to build. That's all. The mechanic gremlin can be appear on a drilling machine (as above), a tank which move slower but shoot cannon over you, and they can control a lumber (jumping on their back and make them move faster), etc...

~ A gunslinger gremlin: Well this gremlin can shoot you with its valliance, callahan, alcheimer or shadowtech alchemer (depend on the tier will have different gun). This will be the most dangerous gremlins.

+Slime: They're second easy (except the @^$& Polyps).

~I suggest a non-effect lichen, but it could turn in to an burning oiler, an electricize quicksilver, an icecube or an poisonous lichen whenever someone hit them with an effect things.

~ A lichen which can move underground (you can barely see it). And it will show an attack beneath you, they jump on the ground and spike you (well you should move non-stop), around it would have spikes too.

+Undead:

~ An invisible ghost who will haunt you and curse you when approaching, the only way to get rid of it is having someone attack it (I don't know about going solo with this one lol)

~ An exploser: A disgusting fat zombie that running as fast as a phantom trying to blow you up, it will blow up when standing near you or being killed.

~ Fever: A dying corpse smelling and being erode which can't move which is like the thorn tree but have elemental effect when you run into them. Can't be killed.

+Fiend: They're too strong and dangerous (everyone of them). What more do they need anyway?

+Construct: They have some monster that associate with gremlins as above (well they're always together, aren't they?).

+Beast:Despite the anoying wolvers and the useless lizard, i have some suggestion cuz they're so less kinds, only two maybe.

~ Insects: Well they're not beast but i want to add this idea. Maybe there's some insect like a Butterflies which can save monster back to fly by sacrifying. A dragonflies that when they see you they will make a straight attack toward you, you can dodge it easily, but not with so many ones.

~ Maybe a snake (not a beast i know), that have several part (like snake game), the other part can be attack and kill but the head can't be( the head will self-explode when the other parts are destroyed). It's perform a biting attack that make them stand still for some seconds.

-Scene:
Aqua: Maybe an aqua scene where the knights explose the ocean.

OK, it's done thanks for reading. And please leave comment. Don't be rude :D Please.