That’s a lot harder to speculate on, but if we assume a relatively direct extrapolation from current space technology, kinetic impactors are still probably your best bet.
Unless technology goes in an unexpected direction, everything I’m aware of says armor is going to be completely useless. Things moving at orbital velocities and greater will go through the heaviest possible armor easily, while the extra mass simply increases the necessary delta-v to make any maneuvers. I suppose one could try to design the ships to minimize damage; make it so that any particular impact is unlikely to damage critical systems by having them spread out considerably, but even this seems to present no realistic defense. So, most likely we have to assume that ‘defense’ consists of trying as hard as possible not to get hit, and the best defense really is a good offense, because the only thing you can do to not die is make sure the enemy never hits you in the first place.
That turns to weapons. Lasers and such may be a thing; we are developing real lasers that actually work to some degree of effectiveness in an atmosphere. In space, with no intervening air to weaken the beam, a laser is much more effective. But, lasers are actually one of the things we can armor against pretty well; ablative armor that basically vaporizes, forcing the laser to expend its energy trying to burn away the vapor would severely diminish the effectiveness of something like that.
So, the old standby of a kinetic impactor. Here we’ve got a few questions: how maneuverable are these ships? Today, our ships lack the maneuvering capacity to evade an intentional attack. In order to make this anything other than another case of MAD (they shoot each other and kill each other almost simultaneously!) we have to assume propulsion technology has made massive advances; we can carry enough delta-v to do serious maneuvering. The vessels have to be actively scanning for any shots fired and adjust trajectory to avoid them. This, also, means the vessels can’t be manned. A manned ship is limited in its maneuvering by the ability of its passengers to withstand the g-forces involved in the maneuver. In order to maximize maneuvering capability, the ship cannot have the limitation of needing to provide a safe environment for a meatbag.
If you’re firing unguided projectiles, the speed of the projectile versus the enemy’s maneuvering capacity tells you the effective range: once you’re inside the enemy’s ability to detect and maneuver against your projectile, you win. Of course, firing that unguided projectile will affect your own speed and maneuvers due to Newton’s third law of motion. The development of unguided projectile weapons will thus be an exercise in trying to make sure your effective range is greater.
Guided projectiles will be effective from a longer range. Further, they can be released from the launch vessel before they use their own thrust to propel themselves, thus launching them doesn’t affect the maneuvering of the ship doing the launching. In space, a guided missile probably isn’t going to need explosives. We’re already talking about tremendous speed; any collision is going to shred the enemy ship, as noted earlier when talking about armor, so missiles are basically an engine and fuel and maneuvering computer. The objective is merely to smash into the enemy vessel.
Then there’s what Stranger on a Train explained: a cloud of chaff or small objects will shred through anything impacting it at orbital or higher velocities. That means that you may not even have to hit your enemy directly with your missiles. If instead of being nothing more than an engine and fuel, your missiles have a payload of, basically, caltrops, all you need to do is launch a missile roughly ahead of the enemy craft, deploying a debris field large enough that their maneuverability does not allow them to escape from it. They plow into the debris and their craft is shredded.
Putting it all together, a space battle between ships at some point in the future when maneuvering capacity has reached the ability to allow either side to have even a chance of ‘winning’ the battle instead of simply obliterating each other, means both sides will fire a massive swarm of missiles and caltrops at each other, whether this be single ships or fleets. Then, both sides will maneuver as best they can to evade the enemy missiles until the missiles run out of delta-v to maneuver with. Also, the more ships and missiles in play, the more difficult the task of plotting a safe trajectory.
The question of jamming or electronic warfare is an interesting one. What does the enemy use to detect your weaponry? Probably a mixture of radar and heat signatures. These could be confused by spraying out a signal specifically meant to bounce off everything in the battlespace in confusing patterns. But, this would degrade your own ability to detect enemy projectiles as well - there’s no way to interfere with the enemy’s detection without also interfering with your own, as far as I can figure.
It thus seems to me that overall military doctrine consists of saturating the battlespace with as much stuff as possible, heading toward your enemy, while avoiding as much of it as possible yourself. Technological development relies on making better computers and sensors, able to track and calculate ever increasing numbers of trajectories and determine how to evade them, as well as whatever improvements in maneuvering can be squeezed out. Also, weapons development largely depends on trying to make your missiles and caltrops less easy to detect, their trajectories harder to plot. And specific tactics would be focused around trying to force your enemy into a location where they’re surrounded by debris and can’t escape.
Every last bit of this will be automated, of course. Assuming humans are involved at all, the human(s) will be on a command ship, and their only real involvement will be to designate whether a target should be destroyed or not, and to give the authorization/order to engage. Once that order has been given, destroying the command ship won’t stop the rest of the ships from executing. And if the automated ships are programmed to immediately attack anything non-allied in the area if their command ship is destroyed, then there’s no point attacking the command ship at all, since all it will do is trigger the rest of the fleet to engage anything it detects. This is probably a good thing for the humans, because the command ships are sitting ducks due to the aforementioned limitation of needing to provide a safe environment for a meatbag. Command ships may well be considered non-targets by treaty, and may be required to themselves be unarmed, since there’s no benefit to taking out an enemy command ship. Tactics are handled by the computers, and if strategy still involves humans, the strategic leaders and generals and admirals don’t need to be putting themselves on the front lines. The command ship contains an officer whose sole function and discretionary ability is to say ‘yup, those are enemies, destroy them’.