1. I feel like the sheer quantity of stardust should be toned down somewhat, dropping less often while having more buying power. Reason being, things currently feel tedious more than they do hectic. Maybe adding more things to do like additional places to spend stardust or different kinds of drops or dynamic hazards from the enemies could help alleviate this as well.
2. Stardust costs seem more or less fine, though the black hole cost is teetering on the edge of being too much, especially on the third stage.
As for tower balance against enemies, the Saturn tower is far too strong. Multi-target towers should always be more expensive and deal very low amounts of damage to each individual enemy. I'd also recommend having a simple single-target tower be the starting tutorial tower once you have one implemented, instead of starting the player out with Saturn.
3. Here's some ideas:
Sun: Absolutely would be fire elemental, though lots of possibilities for how it attacks.
A. A large, single-target beam that melts through enemies. Maybe weak and constant, maybe strong and periodically goes on cooldown.
B. Launching fireballs.
C. Solar flares that pulse out to deal bursts of multi-target damage.
Earth:
A. Bolts of water.
B. Flurries of 'tiny' rockets/nukes that either focus on multiple targets or go to one target and move onto the next if they die before the flurry is done. Humanity going nuts with its bombs or something, could be neat.
C. The Moon constantly orbiting around, damaging enemies as it passes through them. Upgrades would increase the orbital speed.
Jupiter:
A. Single-target lightning strikes.
B. Preiodic gusts of wind that push enemies back.
Pluto:
A. Freeze/slow effect to nearby enemies.
B. Upgrade path where Pluto periodically freezes/slows all enemies on the map for a short duration, but goes on cooldown for a long while. Something something joke about pluto flip flopping its planet status over and over.
C. Pluto and Charon together, launching out icy projectiles (irl they often trade material due to being so close).
- Some kind of asteroid-related tower(s) or something with the Kuiper belt
- A comet tower that costs a lot, has a purchase cooldown, and only fires one attack before destroying itself, but it causes a huge comet to rip across the map and take out all enemies.
- Similar to the single-use comet tower, maybe a meteor shower that bombards the map randomly for a duration of time, lasting for a while instead of being instant.
4. Occasional hazards/obstacles beyond ice, like moving blocks or timed spikes or something of that sort could make the levels a lot more interesting. Alternate routes to areas would be good as well, so you don't always go through the same choke point and have more options in movement. Perhaps some routes are faster but more hazardous, etc.
5. With how overpowered the Saturn tower is right now, most of the time spent is just walking back and forth to grab stardust and fiddle with the selection reticle, lots of laborious downtime between waves. I will say though that standing in the middle of a pile of stardust and spamming E to spew them all out quickly is really satisfying.
6. The UI itself is good, though I'd recommend having some text for the next wave button. The selection system is very neat, though sometimes becomes a struggle when I'm trying to toss stardust (especially in large piles) since it loves to latch onto towers, often forcing me to move things a distance of one tile at a time.
I'd recommend having a setting that lets the mouse interact with things (same as E) if the player clicks on anything other than a UI button. Constantly going from WASD to tapping E can get a bit clumsy at times.
Along with that, I'd love to have a setting which allows the player to, when standing still, use the mouse to aim the reticle on any specific tile they want within range, for more precise control.
7. Not sure.
8. Adding more towers, enemy types, different things to do or focus on, and more interesting level layouts and hazards will go a long way for fleshing out what the player can do and strategize over.