Am I allowed to make a game with the same textures as a previous platformer I made called Cubin'?
I would use new assets. I checked out Cubin, and the game mechanics were good, but there were no animations. Jams are a great place to try new things. I would like to see everyone try doing something new.
As in your case, I would focus on aesthetics, now that you have a good character controller. I would love to see your game have a character as the player. Pixel art is always a good option, also because Gdevelop has a pixel perfect feature.
The grass pattern is nice an erratic. A few suggestions though. As for color I would reduce the saturation, opposing colors with high saturation are hard to look at. Also, grass rarely lays flat on the ground, adding shadow will create the illusion of depth. Also, a highlight on the surface makes it look as if light is coming down. Dirt is also rarely uniform (i.e., a solid color), adding speckles to the dirt obfuscates the uniformity.
Color theory is very complex, and making appealing colors is an entirely separate job. I'd recommend looking at some color palettes. This is the website I use: https://lospec.com/palette-list.
Yes, I intend to. Also, please use the discord to discuss game assets. It's just easier for me to use than itch's discussion board, and you'll be able to get feedback from others as well.
If you don't have a discord account, it is very worthwhile if you intend to make a full game. You'll also get your own channel in the discord to promote your game if you win.