@Marcgfx
Alright this is going to be one of those long posts but I'll try to detail my thoughts as much as possible.
Weapons; this is an action shooting game (not FPS but you're still shooting 100% of the time). Now you say weapons and upgrades require mechanics, I am familiar with that. I know how implementation may be hard. Two things here:
1- You need to identify what is the core of the game? What is the "Flare" is it the vertical progression (i.e. upgrades for your robot)? Or it is a horizontal progression (getting more towers and assistants)?
2- You need to hammer on that part.
Right now I don't know after playing the demo what is the "progression" in the game. Do I upgrade my robot? It feels this way, I get some upgrades. But they don't feel amazing that I think it isn't the real hero here. Maybe it is something else. It isn't the towers or getting companions either since I only received one of either by the end of the demo.
So as you can see there are two options (in my opinion and off the top of my head) have slow progression but each tier means something. When I add HP to my robot, I am adding 200 HP not 10 HP. Or have it fast and not that effective. Every 5 minutes, I get a +10 HP or something. This makes things feels relatively good. You're progressing meaningfully not just chipping away in a survival game where by the time you get a house that doesn't fall apart, you'd have put 100 hours in the game.
This means if you think mechanics are hard to implement or an issue to do so for whatever reason, then maybe just make them stronger and maybe introduce them a little faster.
Maybe even just change the drop rate to be more frequent and things will feel better. Right now I got only 1 upgrade per level and sometimes I didn't. So yeah...
Now let's take the X enemy dies by Y gun. While this is a nice mechanic, it doesn't feel good. Why? Because I have to suffer from it. The nuke/pulse recharges every M amount of time and I am stuck shooting peas at an enemy that won't die otherwise. So picture this. I have 5 enemies on the screen, each of them die fast to pulse/nukes. They are 5 different corners of the map. I have to kill them all with that or spend 10 seconds shooting at them like they are meat bags. If I use my pulse on wait, I am waiting for about 5-8 seconds till I kill the other. I am penalized for using my special attack that kills them and I don't find any other way to do that. You treated the weapon as a special ability and had enemies that require it spawn often (not too often I know) so when I use it to get rid of one, I am left without nothing. Do you see the opposing directions in this?
Which brings another question, you said upgrades are mechanics. Now let's take the nuke vs the pulse. They are (mechanically) the same exact thing. In fact they should be using the same code. Albeit the nuke has a delay. The pulse and the nuke have a target. The nuke has a player chosen target while the pulse a constant non changing target which is you. Maybe the nuke has a movement part (albeit it could be just an animation mind you). So all in all, one code, 2 powerups. What about a 3rd version? Maybe like throwing mines are you. Same deal but now mines need another mechanic but it is a simple "enter trigger area explode". Which means now you get MORE variations since you already have the code. Now I don't know about which engine you're using (guessing Unity or GM:S) but if it is Unity I can tell you this is all pretty easy to implement. You did that already (nukes and pulses). So maybe just think of slight variations that don't cost a lot for each of the weapons/upgrades?
Feeling lost is another issue, quite a few things aren't explained like the last two upgrades in the upgrades screen (the one that appears between level). I don't know what they do. Sure I understand the English part but what is it talking about? You said you'll take care of this so I own't spend time on this.
That grazing mechanic... I never knew it existed. In fact I got hit by bullets and lost HP so I decided I'll just stay away from bullets.
I don't know why it is impossible to give the game a fast lift? Also I think it relates more to overall aesthetic and what is the art being used for. Now personally I am not a fan of this style (no matter the game). BUT, I find something like the black stuff not belonging in terms of art style.
Another thing you could do is as you said just get an artist to weigh in and make it more uniform. Choose a nicer font, maybe replace the color palette a bit maybe? Not sure.
What I can however be more concrete about is polish. Here are a few suggestions:
1- A bit of screenshake when I am firing my bullets
2- When I get hit some screenshake and some effect on the screen that isn't as simple as making the sprite go white and back again. Something more. Maybe have some kind of parts fly out from behind me? Maybe have a knock small knock back? Maybe do something more on the mechanics side that when you receive a big hit or large damage in short period of time some EMP/Shockwave fires and stops/throws enemies away?
3- Have my movement more interesting. Maybe some track on the floor? Maybe some puffs of smoke behind me? Maybe make it more like a robot with the animations so my "track legs" can't move until my body turns to the direction of movement first. Maybe have the tracks turn in place for 0.2 seconds before it actually starts moving. Maybe make the robot heavier and the field smaller?
4- Spiders hitting me don't look like hitting me. They barely animate. Maybe add some kind of "tell" before the attack and make it more impactful. Have them glow or shake or something like that.
5- Getting hit by the black stuff doesn't differ from getting hit by a small tiny 1-hit robot. Maybe have it where if I get hit by the black stuff I get knocked back. I get paralyzed for 0.45f of a second. Maybe make it when the black stuff do attack that the screen shakes. Buffs of smoke or some kind of effect fires when they make an attack with each attack depending on power generating a slightly differnt version of the effect.
6- The core getting hit doesn't really feel like a big deal and this is due to lack of any feedback. What if they get hit you actually get the core to shake? Or maybe it changes color in the parts it hits (not just simply a red hue, something that feels nice). Maybe you show a shader on that part like a shield being cracked or something.
7- Some screen indicators that show danger when you're moving around so when you're getting hit at the core you get an arrow or a sign to tell you "hey, you spent too much time on the right side killing 20 spiders and now there are 40 on the other side waiting for you".
8- Then there is music and sounds. I can't really say I felt impressed. Not gonna say it is bad. But it is OK. I mean I don't even remember what they were at this point. Maybe pass on that and see if you can get better sounds or maybe better music?
9- When that ship drops me or takes me out of the field. It feels rather "plain". But what if you can get some kind of shake. A huge "THUD" kind of effect when you're landing "you're the saviour" kind of thing.
10- With the pulse. It feels like some kind of sprite expanding and that is about it. Then everything disappears. What if it makes some kind of ripple (shader?) on the ground with some robots flying left and right (from the shake wave) as they get destroyed and you're stuck in place for (0.25f) seconds or something form the awesome power you just used? Maybe add an ever so slight screenshake.
I can probably go on more and more, but these are all suggestions. Some may work, some may make sense. Some may not. I don't know what works best for your game and I know it isn't a very interesting thing to do overall (I know I always want an artist just to deal with it and tells me when do I call .Play(); on them
). But I am pretty sure the feel of the game will be drastically different if you implement some of those things or your own versions of them.
Lastly what makes your game unique? So far that one enemy that you introduce once at the end of the level. So you make me go through some mediocre experience up till the point where I get something interesting. This ends in about 1 minute and I am back again to the "oh, this is like other stuff" experience. Why am I not fighting the black stuff from point one? Why don't the spiders get replaced with something more interesting. Everyone tired of fighting spiders -- thy are in every single RPG. Why not make a small weak version of the black stuff? Heck make the spiders from the black stuff and have them behave in interesting ways.
What is unique about me? What is unique about the base? What is the unique about the towers? Now you don't have to make EVERYTHING unique, but you need enough "unique" to sell it out. Right now that is enemies and you're not capitalizing on that, so maybe either introduce something else or capitalize on that and introduce one more unique element that doesn't have to be so prominent for example.
Example take take Hollow Knight. It isn't exactly something new is it? It is uniqueness (in my opinion) is three things:
1- Visuals -- they are just amazing.
2- The overall setting, it seems to appeal to some (certainly not me) and how deep the lore goes with its various connections.
3- It is difficult gameplay; a semi-souls game in a metroidvania coat.
Can you list yours? Are they as unique as the above 3? Are you capitalizing on them like the above 3?
Sorry if I seem harsh or attack-ey. Trust me. Not my intention. But I thought it is better to lay the whole thing and be as honest as possible and as blunt as possible. I feel this will be far more helpful than just try to embellish every word I say.
Hope this helps in some way.