Hey there
Here are some Ideas 4 your Game (5/5) *.* I just write some Keywords to keep it short.
2nd Mission is cool. Its a really a nice variation. Also I like the Enemy Type. (pesky worms ruin my day) maybe a larger enemies that spit acid would spice things up - maybe later in the Game.
Weapons:
Melee Weapons 4 the mech like a sword or an axe plus a shield
Pistols, Uzi, Desert Eagle maybe with Akimbo possibility
mortar 4 the Mech (swap 4 missels)
possibility to lay or shoot mines
drop of a stationary tower or repair tower
weapons overheat
reload weapons
Vulcan gun is pretty op I think
Armor:
with a small energie shield that regenerates slowly after hits
with a jump Pack
with a radar pack
that spwans drones from time to time (repair drone, combat drone, defense drone)
Grenades:
stun or flash grenades
New Abilities:
call in an airstrike with cool down or can be collected on the map
or an emp strike
transform into a helicopter 4 a short time
Global Achievements:
Kill 100 enemies from this type or that type
Kill 100 enemies with a special weapon and so on. Maybe to unlock to new weapons or to upgrade weapons later
Level System / Collect Resources:
to level up weapons or the armor to play higher difficulties
would keep up the grind and motivation 4 the player
Enemies:
Boss Fight in the final level
or Mini Boss Fights after every 5th Level
Enemy factories that spawn new enemy tanks etc.
i know developing a game takes a lot of effort and many of the points above may not be possible or not a top priority
these are just a few thoughts and maybe they will help you to create a really nice game cause there is a lot of potential in this
pls keep going and dont give up <3