Designing those sorts of things takes nothing. A good programmer could add in destructible walls and objects in a day or two, in all likelihood
That's complete bullocks. For something properly done for a modern game, you would need a very very minimum of 3 weeks for a basic implementation for 2- 3 coders, preferably one of them senior, much more if it is done late in the dev of the game (i.e. the game was not coded around that). I am speaking about destroying whole sections of walls / structure there, not punching a door-sized hole in the wall JA2 style - which would still take a couple days minimum.
In addition of the basic "wall disappears when hit by big boom thing", you have to implement a lot of additional stuff :
- Resistance of the different type of walls, and "wall-destruction" capacity of weapons. Sure, your sniper rifle can inflict 1000 damage to anyone it hits, but it won't destroy a wall. So you need to revisit all buildings and weapons, add a new parameter. If done late, it can take a lot of time.
- Chance of collapsing. Surely you don't wall the 2nd floor of a building to "float" if all the walls of the first floor are destroyed. You don't want either one section of wall to hold a huge area, for instance if the only remaining section of the wall is on the exterior west wing of a huge U-shaped building.
- Effect on collapsing on anything below. If a mirador falls on your tank, the tank should survive fairly well. If a 15 floors building, the rumbles should occupy several floor themselves, and your tank is lost. On the other hand, Mirador falling on a guy
=> Not good.
- The tiles are collapsing, now what happens to all the decorations / characters / vehicles ON the collapsing tiles ? What if you tank was on the second floor or a 2-floor building for some reason ?
In addition, your GD better be good, because the impact on the game will be huge.
Basic problems for a GD :
- should building collapse be checked and done during a turn, or should they be checked for collapsing between your turn and your opponent turn ?
- how to avoid the player just destroying everything from afar, like you do at Blitzkrieg when you have arty and not your opponent (UFO solution : civilians and weight of equipment ?
- what to do if a target is made unaccessible ?
The AI should be improved significantly also.
After this "basic" implementation, you have a huge amount of debug, balancing, test to do. And then you also have the issue with creating all the ressources for the different "stage" of a wall (intact, destroyed at the minimum) + rumbles + many other isues.
UFO allowed full destruction of buildings (which was cool) because back then it was acceptable to see the second floor "floating" above the ground, and of course the last alien inside (happened to me). In a modern game, this would NOT be acceptable.
As for the game being rejected because it was too close of the original, this is not the main reason if I read correctly :
"It's hard to even describe now," Solomon says. "I don't know what I was thinking. It was the original game, and then over the top of that I had put ... soldier abilities ... a cover system ... new alien abilities ... new weapons. It was ... incredibly complicated — not complex. Complex is fun, complicated is bad. This was a very complicated game. It was more complicated than the original."
In two words - feature creep. Not good for a game complex already whose target is not grognard. See Wake of the Gods for Heroes III. Each individual addition is cool, but the total is bad. It is hard to admit, but the game might be better currently (and yes, there are several things badly lacking - including of course total destructability, and randomness) than in its "first Salomon" iteration. Remember that by that moment, the randomness of the levels was gone already.