Now all of this is already in game. However the Pie shaped LONG RANGE projection now makes Star Bases an offensive tool as they should be.
Why should military starbases be offensive tools? Regardless of the shape of their area of influence, a military starbase is only going to be offensively useful in a very narrow window of the game, before and after which they're back to the current situation of 'wasted investment.'
Additionally, military starbases are analogous to fortifications. They're relatively costly investments of resources (and time, if you don't have the ability to pump out a lot of constructors quickly) that make an area (theoretically) easier to hold and are only offensively useful if your offensive gets bogged down in an area long enough for you to build up a starbase in the region as a kind of "siegeworks" (this situation is the siege/trench warfare analog of Galactic Civilizations - fleets fighting back and forth in a system with the balance not really swinging to any particular side enough to either drive back the aggressor or allow the aggressor to achieve the goal of invading planets). At the moment, they're fortifications that don't really boost the defenders sufficiently to be worthwhile and which are perhaps excessively vulnerable to destruction.
Adding defensive bonuses to owning players/AI's fleets either defending or attacking.
Penalizing enemy fleets on attack values but 5, 10 or 15%
Penalizing the offensive power of an enemy is more or less the same thing as enhancing the defensive strength of an ally. Pick one, don't go for both. Also, penalizing the attack scores of an enemy begs the question of how, especially if it's not hampering your own vessels similarly. Unless you're more or less right on top of the starbase, jamming and other electronic countermeasures would presumably be more effective if they're originating from your engaged ships than from the starbase, while some kind of magical laser inhibitor field probably shouldn't be capable of distinguishing between friendly lasers and hostile lasers, nor do I see a good way to justify a field which selectively drains kinetic energy from mass drivers.
One of the issues that I feel that military starbases currently face is that in order for them to be useful, you still require roughly the same fleet commitment to an area that you would have without the starbase - you still need to have a ship or two at every planet to prevent unarmed transports from making landings, and you need at least one fleet in the area to protect the starbase and handle any combat fleets your opponent sends in to clear out the ships you have guarding your planets against invasion (alternatively, you can have enough fleets in the area and enough of a buffer zone that it's very unlikely for an enemy to be able to slip a few transports on to unguarded planets). You'll probably never be able to drop that mobile fleet, but a starbase that can allow you to pull the ships you have sitting on planets? That has the potential to be a worthwhile economizing measure, at which point you really just need to balance out where it becomes worthwhile to use a starbase to provide the planetary garrison fleets as opposed to using real ships for them; you could also have this point shift around based on the size of the garrison fleet, with starbases being more attractive the larger the garrison fleet you wanted gets. Conveniently, this would also give you a reason to go after the starbase rather than ignoring it; after all, if the starbase is a single point of failure for a number of garrison fleets, it could be more worthwhile to commit forces to removing the starbase than it is to commit forces to wiping out each garrison separately.
It'd probably also be good to give them some kind of bonus which applies regardless of whether or not there's fighting in the area, perhaps an approval bonus due to the government's demonstrable commitment to protecting the area, or enforcing a minimum approval level within the starbase's area of effect, or maybe a bonus to the amount of production going to any shipyards in the area of effect (e.g. if there's a shipyard in the military starbase's area of effect, then that shipyard has its production rate increased as though all the planets within the starbase's area of effect were contributing 5% of their unmodified production to the shipyard; this probably should not be something you could stack), or maybe ships passively gain experience within the area of effect or some such thing. The bonus production for shipyards thing and the approval bonus thing might cut into the role of economy starbases too much, of course.