I'm sure some of you guys know about that game called Celeste. Not that I played it, but I watched a couple youtube videos of it, and I thought that the level design, and character movement from that game could work with Plazma Burst 2.5.
This is the game I'm talking about:
https://www.youtube.com/watch?v=Ok9SCAMeoGI&t=882s
I would suggest making the map levels about 3 times longer. Add save checkpoints and "life" points. Dying will result in losing a Life point, and you will respawn in the last checkpoint, losing all lifes will require you to restart the level completely. You will regain those lost life points back everytime you restart the level completely.
Difficulty levels could differ in amount of Life point you get when starting a level: Easy - 4 or 3, Normal - 2, Hard and Higher - only 1; That means you should play carefully, since in hard or higher you wont be able to respawn in checkpoints after dying, only completely restarting the level will be available in those high difficulty levels.
Just like in that game Celeste, make the Plazma burst 2.5 levels rich in traps and challenging in terms of parkour. So the levels will not just be a boring linear corridor shooter like the currect Plazma Burst 2 campaign is. Yes, Plazma Burst 2 had some parkour parts in It's campaign, but It's really lacking and very poorly made.
For character movement, I would suggest make the characters move at a little slower pace, and make them less awkwardly bouncy. Give the option to dash, slide and stick into walls like in Celeste.
And lastly, the AI, oh god It's terrible as hell and dont provide any challenge.
Make the AI's being set in squads with a commander in it, which should able to give orders. The AI should be capable of performing challenging tactics against the player, such as flanking, camping (And shooting though thin walls), scouting and inform on player's presence, and even sometimes to highly prioritize on reviving It's dead allies, and other tactics that Eric Gurt might think of.
Speaking about squading, the game should have a squad system if you or the AI enemy command have AI allies on It's squad. The player, or the AI enemy commander, would have the ability to give orders to It's team mates or allies, like follow me, hold this position, charge, get in cover, prioritize targets, shoot on a specific spot, prioritize on playing defensively and etc.