Thought I'd just write for fun instead of moping about life's little annoyances, which almost all the past posts on this blog have been about. As I'm looking at game design as a hobby, it should come across my thoughts what should be done to improve existing games that I play - this will be a highly relevant question if I ever become a game designer. As this is just a start, I'm not going to write about seriously flawed games like Anthem or Warhammer: Chaosbane, as I'll need tens of pages to write about their problems.
Warframe
Firstly, enemy design - specifically, attacks of most enemies in the game. Basically, Warframe as we know it is chock full of hitscan, homing, or fast projectiles that are effectively unavoidable. This design is intentional but lacks practical diligence: while it may be fun for the first hundred hours, eventually the lack of skill-based interactions makes the game incredibly grindy, given that Warframe has thousands of hours of progression. This reddit post best describes the issue better than I do - I highly recommend giving it a read.
What the post doesn't explain is how important this is. Many other complaints I hear from other Warframe players (frame balance, scaling, etc.) actually mean very little if enemy design is crap in the first place. Regardless of how 'balanced' mob scaling becomes, if enemy attacks lack interaction it will remain the same: in early levels you can run anything you want and the mobs tickle you, making it fun at the start but gets super boring after a while; in the higher levels things will always one-shot you and there is no counterplay aside from running frames with heavy damage reduction, crowd control, or avoidance abilities such as invisibility.
Second thing I'd change is the armor mechanic - it's almost as bad as the attacks as you're basically forced to run armor reduction mechanics (such as Corrosive Projection) to clear harder levels quickly, greatly limiting customization and wasting precious effort put into the game (it makes other aura mods less relevant for example). The problem is you can't change the armor mechanic without doing the first bit and changing the attacks, as without armor most frames would just get one-shot all the time. A good change would be more like this: armor as we know it is replaced by flat damage reduction per hit, hard capping at 80% DMR per shot / hit. All mobs at level 10 or higher will have a base armor amount that increases with level, and is reduced by existing armor reduction mods.
Grim Dawn
I'm not going to talk about this extensively as this blog is (technically) public and I want to keep my ARPG ideas quiet. But basically I think old-school "Diablo-style" zombies are actually poor design, and ARPG developers have been keeping it in their games solely for the 90s nostalgia. The early levels of Grim Dawn are absolutely crawling with these slow, braindead creatures that are equally as braindead to fight. Actually good game design would make zombies more to the fast-moving style of Left 4 Dead (or 28 Days Later if you're more familiar with movies).
Secondly, I want more craftable gear at the endgame - legendary item drops in Grim Dawn drive me insane. Even if the crafting requirements are horrendous or require an equally random recipe scroll, having that alternative over solely relying on monster drops helps players feel more comfortable with grinding.