I'd love it if I could play Proteus co-op. However, this would be a big change, as the game is first person, and you can't see yourself. But if there were another player, you'd probably have to have a visual representation of said player. This could be problematic, as the environment is so carefully crafted and balanced that a potentially erratic element of another player could mess up the vibe. But maybe it wouldn't. I'm going to proceed as though co-op play is an option.
I should mention that it seems pretty clear to me that the intention behind Proteus is to deliver a solitary experience, somewhat reminiscent of Transcendentalist writer Henry David Thoreau's: Walden. However, these ideas aren't really for Proteus itself, which is fine just the way it is, but ideas for an imaginary sequel or alternate game, inspired by the gameplay and design for Proteus. Moving on- with multiplayer Proteus ideas...
Based on the way that the game of Proteus chooses not to say anything directly- no dialogue, no score, no clear goals-- I would want to copy the "journey" style of online co-op play and have no direct chat, only different sorts of audio and visual signals you can relay to one another. (jumping, spinning, rolling of the third type) Not to mention, in Journey, there is no co-dependence. In that game, the other players are just there to work in tandem and keep each other company. I believe this element would honor the gameplay of Proteus while expanding and changing it to include a second player.
Or perhaps the players would be invisible to one another, and could only be seen by their effects on the island. Causing frogs to jump, stones to hum, and the like. This could help players to create a sense of mystery and randomness for one another by indirectly affecting the other's play.
I suppose another expansion on the co-op idea would be the ability to lay out easter eggs for friends. To create a custom island with player-designed easter eggs. That would be a nice thing and fun, too. Like a greeting card. Examples of this element include The Anna Anthropy Occupy game - Keep Me Occupied, and Inside a Star Filled Sky - both these games allow the player to plant flags.
One more thing I would like to see is expansion of the island area in general. One of the most exciting parts of Proteus is when I realize that I've found something I didn't know was there. However, by now I wonder if I've seen all there is to see. It seems to me that I've discovered the outer limits of the game, so now I'm left with the inner limits, so to speak. I'd like it if the outer limits seemed a bit larger, to preserve the mystery and sense of expansiveness. If additional players were included, this may become a requirement.
I suppose when introduced to a new and desirable gaming experience it is only natural that I should want more, more and more. Most especially, I want to share Proteus with others, it doesn't even have to be my friends and loved ones, just anyone who will appreciate it. I think that Ed Key and David Kanoga really achieved something great with this game. So, despite these ideas for changing this game, I don't have any expectations that there will be a sequel. However, I do expect to hear some 'inspired by' comments with regard to Proteus, and that suits me just fine.