I'm adding on (you could say) to AmazingAlexx's suggestion when I say this, but there should be some sort of new currency for a casino. Maybe have a blackjack table that uses the power level of everyone's cards shuffled in, as well as some other ways to use it?
I'd really like a way to search for people in multiplayer. Maybe have categories that separate people according to their level, or even a search bar. Maybe implement some sort of clan system so people could have 3v3 battles? Possibly have free-for-all where four people are playing against each other somehow?
I think the ability to earn some type of money during campaign matches would be awesome. Along with a hat for the two with the top score. (Campaign matches would be pre-set matches that you couldn't change, thus making it harder to obtain the hats.) The money could also be used to upgrade your character before a match, upgrade his/her stats permanently, buy weapons in a match that you normally can't obtain, be able to buy different maps, etc.
I can't wait until, or if, there is a GDITS 2! Then you could possibly have a leveling system with separate monsters. Like, if you play as an Astronaut for a while, eventually you will make him more powerful.
Either one of your temples was in another's range, or you just didn't save. Both are your fault. Also, you can sacrifice more towers by upgrading the range, even thought you can only give it so much before it is useless to give it more.
This game is absolutely awesome! The only thing that brings its greatness down a bit is that my internet stops responding every once in a while when I play this. Otherwise, great! I would recommend a level system, ranked matches, skill points in damage, health, movement speed, defense, or even how much damage certain weapons do. You could expand from this into so many other things.
---My Personal Suggestions---
(1:Each monster you collect should have a level cap of 10, and killing other monsters should give you EXP. Each level up then gives you 2 skill points to put in either your SPEED, DMG, or HEALTH. With this you could sell packages like a $1 pack to give you +5 skill points for 5 monsters of your choice. This would also encourage ranked PVP matches.---Please + if you like the suggestions.---