definately wouldn't want to see the cost increase for getting an easy 8 called in every time or a fuel upkeep it is hard enough getting a couple late game now that there 145 fuel.
Child please! Don't think it would be super-difficult to adjust fuel cost and CP requirements.
NO.
just add fuel upkeep relic!
Wouldnt only encourage saving for late game call-ins? Don't see how that would fix any balance or gameplay issues at all.
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I agree with OP, simply put tech-requirements for Doctrinal tanks would fix the horrendous call-in gameplay that is so prevalent right now. How much more interesting wouldnt it be to watch replays and casts if this was the case? "This guy went T4, will he get Panther for AT, Brummbär for AI or Tiger for something with a bit of both?"
You actually have to use your timing to choose when to tech. You can even scout enemy base to get an idea of what he might get! "He went T3? Then he probably won't afford teching t4 later to get his Tiger."
"Oh fuck, I know he has ISU commander and I can see him building T4 now!"
Other units could do well with a maximum amount or very long cooldown at the very least. (I'm looking at you ISU/Elephant!)