Hmm... Interesting idea. But it might be hard to make it viable though. I assume that you mean something like "You may spend IP(s) to attack other player(s) 'close to you' which lose some amount of hit point. If their hit points is 0 or less, the unit is removed from the board"?
Currently it is very cheap to make a new unit, and starting to attack someone will probably have serious diplomatic consequences. Somehow you want to balance that, which is quite hard. But I think it's a good idea if executed well. Could you give some more details of your idea?
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I had an idea of something which I consider to be a nice new rule: you can spend money to gain additional IPs on your turn (or maybe also on others' turns). For example, the first extra IP could cost N$30, the second N$60, the third N$90 and so on.
Just wanted to say it, maybe someone doesn't have an idea for a new rule but likes this one, or perhaps I'm going to propose it myself on my next turn.