Oi! Don't knock Attacktix!
Seriously though, turning Monopoly into a war game would further complicate the rules (Monopoly already takes a long time to play as it is) and it would also turn off some people who just aren't into that sort of thing. I suggested using Attacktix because the rules of battlefield engagement in that game is really simple (i.e.: launch missiles and try to knock the other piece over) - believe me it
sounds simple but it actually takes some technique and strategy to pull off well in actual play.
The optional TF Monopoly rules allows you to attack any other player's property so long as they don't own the entire colour group for that property if you roll a double-6. Once you've challenged another player you both roll a die. The person with the highest roll gets control of the contested property. It's a simple and effective way of having "battle" in the game of Monopoly without adding a stack of combat rules to it.
There are other TF battle games already out there if you want to play something more combat-oriented - but I think that it would seriously disrupt the flow of a Monopoly game... and quite frankly in such a long game that would be the last thing that most players would want.