Everyone wants this game to get bigger it seems like, and heres my suggestion on how to do it. In keeping with the Doom theme, maybe have episodes, like the original game had. The current set of 25 levels, with unique levels included, could be Episode 1 - Knee Deep in the Dead. Have another set of 25 levels with different unique stages put in, and maybe some different monsters. For example, in Episode 2, maybe have former commandos appear from level 1 onward, maybe have the Spider Mastermind as the boss. Or make the current set of levels episode 2, and make an Episode 1 which has easier monsters for the most part, maybe leave off commandos, Arachnotrons and Arch-viles or something, and have some beefed up Barons as the boss. And reduce the weapon availability so you can't get plasma rifle or BFG. Then for episode 3, make that more Doom 2 like, with harder enemies and bigger rewards. Maybe make JC the boss of that or something?
I'm not sure how the game is coded, so I don't know how hard something like that would be to implement, but it seems like it shouldnt be too hard. I suppose the hardest part would be game balance.
And for my second idea, this one is simple. A Packmule trait, which would basically allow you to carry more in your inventory. Maybe get five extra inventory slots per level, max it out at 2 or 3. It's something I'd like to have later in the game when I want to stock up on ammo, but have no room.
And if you want to keep inventory all on one screen, simple double column it, it looks like there's plenty of room.