An RTD based somewhat off of Amorphous Plus mixed with the concept of a survival game. Each player gets to choose all kinds of options, determining how they attack, what specials they have, and what role they play in the party.
The first main choice is your Main Weapon: you can always take the gigantic sword from the original game, which is a versatile tool that lets you attack at close range and hit multiple enemies at once, however, opens up for counterattacks.
Other weapons may afford you some range or not stun you on swing, allowing you to keep your defense up at all times.
Three specials may be chosen, however, "specials" encompass not only active, summon-able benefits, but smaller boons that can be attached. You can bring a single turret- already powerful enough to kill things in one shot you need three or more hits with the sword to take down- and then choose to enhance its ammo storage and targeting software (The original turret can only take so many shots, and values every target to have equal priority, and is capable of hitting players who target the same thing as it does. Advanced targeting software automatically lets it aim at more threatening targets, and will never aim at something a player targets.)
Things can kill you easily- however, you have a system similar to HP in "Endurance". Basically, this is a measure of your luck- does that Biter lunge and come just shy of eating your shoulder, or hit you in the chest and maul you to bits? Each enemy attack reduces it, and it regains very slowly. If you are Stunned, however, you cannot react and your Endurance caps at 1, effectively making you a tasty, helpless morsel.
The Sword stun is a special case that simply cuts back on Endurance, as it would be useless otherwise.
Lastly, you gain an Ability. An Ability is some innate skill you have- a common one is Revive Teammate, but it should be noted that a good portion of deaths are nonrecoverable- acid will end you. Others, such as Hercules, negate weapon drawbacks, or Technician, which increases the speed that specials regen and gives you a free upgrade slot.
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I'm not sure how this would work in tracking the actual beasts- a 2D map is a great function but may be too involved for the likes of an RtD, but it would nonetheless be very fun. A simpler option may be gladitorial-style fights in sequence?
I may or may not try to rig up a roll20 map for something like this- all you really need is four colored player markers and some blobs.