When I was still thinking about possible games, I haven't even thought in that direction. Your idea is extremely interesting and I think you will make a highly competitive game in which the players can acquire a high skill level. I look forward to seeing your progress! :) 

I'm a little worried about the crystals. I think you should focus on your afterimage/copy mechanic. This is your main focus of the game and I find the crystals to be an unnecessary distraction at the moment.

My idea would be that over time you simply let more copies follow you, so it is foreseeable that the game will become more difficult and at some point, there will have to be a winner.

Love the game! I think it has a lot of potential and I'm curious what you will come up with over the next month. Especially the idea with the crystals to enable the past-ghost-tracks is great.
I feel like the trailing ghost can add a lot of visual clutter.
Maybe don't show the trail at all and just reveal the action performed at that place once the crystal is placed. So both players would have to remember who did what where.

Pro: The idea and the duplication mechanism is super awesome!
Con: I think it would be a little challenging to make the visuals neat, too many actions can be overwhelming.
Suggestion: Try to find minimalistic design for characters and actions.

Pro: I love couch coop games and this one sounds like a lot of fun already. You made a really interesting duplication mechanic that is not too complicated to work with but leaves plenty of room for creative use and nuance.

Con: Adding addtional NPC enemies and too many enviromental modifiers in an arena could create too much chaos and visual clutter.

Suggestion: Games like these thrive on being chaotic and fast but that is usually already accomplished by setting up mutliple player against each other in a free-for-all. I think it would be better to focus on having one interesting set piece an arena revolves around. Your items and the players will already create enough dynamics to keep things going while they are playing around the set piece.

The duplication idea is unlike any other implementation I've seen in any game. It also enables a very fun gameplay style.

However I feel like the game would have too much chaos and would be unplayable if enough time passes and there are past duplications everywhere.

Limited the amount of duplications should solve this, and also improves performance. Making a multiplayer game with unlimited duplications would be very challenging. You can also make powerups that to have more past duplications.

Pro: I love battle arena games and their competitiveness. The ability to use your past self's attacks would certainly make the fight more interesting and challenging. I also really like your visual art style.

Con/Concern: I imagine that having a lot of Phantoms at a time would result in visual clutter and would make the game chaotic and less enjoyable.

Suggestion: I would suggest limiting the number of Phantoms a player can have or making the whole duplication an ability that will be active for a certain time and then must recharge.

Pro: Yeah! This is the first idea that comes to my mind when I hear the word duplication. I really like this game idea. I have seen some puzzle-platform games based on the Chrono, like Braid, but never thought of using it in a battle arena.

Con: If the lifetime of duplicates can not be appropriately set, the game scene will be chaotic.

Suggestion: There are some tutorials and tools to record, rewind, and replay, a plug-in called Chronos may help (you may already know that). I would say I'm really looking forward to seeing your final games.

Pro: The game idea is out of the box. It is opening a potential for a very competitive and great game. Introducing also a demo in the presentation was amazing to visualize your idea. Good job on that.

Con: Thinking about an arena game with 4 players each having multiple afterimages would overwhelm the arena very quickly. If I understand this right, we can see around 30 different afterimages in the arena, especially if the players place the crystals very close from eachothers.

Suggestion: The afterimages can be invisible as long as they are not activated. As a player I would not care for the deactivated images anyways, only the active ones I should attend to.

Pro: VERY AWESOME GAME IDEA! I really like that there is an ability to use your past self attacks this makes the game play really interesting.  

Con: I think there will be times where the scene is just overcrowded and there is too much going, also the models you showed for the game visuals are quite challenging to design 

Suggestion: Somehow limit the number of duplicates I could have or introducing maybe some other game idea just to not have too much or to many objects happening in the scene 

Pro: I really like how deeply you thought about the idea of duplication and how the player can retrieve actions they did in the past, the game idea is really unique.

Con: I'm not sure how will the scene be handled within certain time frame if there are a lot of clones in it, is there like a threshold in order to limit the number of clones?

Suggestion: The visual inspirations are really artistic and it would be awesome if they are developed similarly. Maybe they will need some time, so it will be better to start with them in the beginning to improve them visually along the way. Also, as the scene might get crowded with clones at certain time which might confuses the player, therefore it will be really helpful if you included introductory level in the beginning. 

Pro: The game idea is cool and refer to super smash bros by Nintendo, this kind of game will bring lots of entertainment when player could play it with his friends.

Cons: I am not sure but just doubt that if the game will lag or become slow if much clone effect on the battle field.

Suggestion: If multiple duplication happens at the same time periodically, then the game effect sound must be noisy because of the same sound pattern. Try to find a way fixing it.

Pro: The mechanic of using a past version and action of yourself to defeat others sounds fun.

Con: The amount of cloning might clutter the arena too much especially if it is a small arena.

Suggestion: Limiting the amount of clones and using bigger arenas might help the players.