Stellaris

• The following assumes you have all Stellaris DLC because a) it's all cool and good and worth it and b) it's a mild pain in the ass to remember what is / isn't DLC mechanics in these kinds of games(only exception being the Nova / Galaxy Edition upgrades, as those are literally just a single portrait and some eBook nonsense).

• As of 2.0 the way to expand your territory is to send a construction ship to an unclaimed system and build an outpost there. Then once it's claimed you can set up your mining / research stations and colonize any planets that are currently colonizable.

• You don't HAVE to claim your territory contiguously, but you get increasingly higher influence costs the further the system is from any of your claimed systems, so the bulk of the time it'll make the most sense to always grab a system adjacent to your empire's borders

• Even basic Outposts can have some Defense Stations built around it without upgrading it to a Starbase. Handy if some small-scale nuisances like pirates keep up gunning for the same system.

• Speaking of pirates, they pop up in unclaimed systems adjacent to your claimed systems. They also increase in chance if they have multiple claimed systems around one unclaimed system. So this is an incentive to eventually fill in any unclaimed bubbles left over in your territory that you might've skipped past initially because the system itself wasn't super valuable.

• Don't sleep on Starbases and their upgraded forms though. They are great for defensive wars, especially when you get upgrades like FTL Inhibitors that forces the enemy fleet to take care of it before being allowed to move past that system and into your more vulnerable territory.

• Like a lot of 4X games grabbing more territory will increase the costs of obtaining new technologies / traditions, with a colonized planet increasing the costs a lot more than a system. You can see the exact percentage by hovering the tooltip over the current cost of a technology / your next tradition unlock. This isn't super crippling or anything but it's something to keep in mind when it comes to what buildings to construct and whether or not that 2 energy and nothing else system is worth claiming.

• The priority of what systems to grab: systems with colonizable planets(either immediately colonizable or colonizable soon-ish), systems with high resources, systems that make good chokepoints because of how the hyperlanes are situationed (either for waging war or cutting off nearby neighbors for expanding in a certain direction you'd like to expand), filling in space to not have to deal with so many damn pirate attacks

• The general agenda of other empires can be deduced by their trait description. One who's a "Spiritualist Seekers" will like other spiritual empires, and a "Fanatical Purifier" is probably going to try and kill you at some point. Gifts and deliberately favorable trade deals are usually the only way at first to improve relations, until you can get a treaty which starts building trust.

• When you start making Sectors, look at their planets and see if they lend themselves to a specialization rather than just generic resources. They may have really good energy planets for instance.

• Lasers beat armor, kinetic weapons beat shields, missiles beat a little of both and are more accurate, but can be shot down by point defense. Normal guns and lasers are not point defense.

• Energy production is important, even if you have a good surplus throughout the game, a sizable late game fleet can put you in a pretty big deficit if you don't have a large production. Keep in mind that a fleet costs a lot more upkeep when it is not docked.

• Energy Credits are primarily used as maintenance costs for ships / buildings / stations and for buying new leaders. Outside of that though, some not immediately obvious uses for bulk Energy Credits: getting bonuses from the various Enclaves(they're sorta like Civ V/Vi City-States; Curators will give boosts to science or buying new scientists, Artists will gives boosts to Unity, Traders can trade your energy for minerals), paying off Raiders to attack enemy Empires, Campaigns in the Policies and Edicts tab

• Research is stored when your scientists are busy with special projects so don't be concerned about falling behind on tech due to projects.

• Look through the Ascension Perks tied to your traditions at the start of the game, and try to pick a few to go for. Some are locked behind tech, so its helpful to decide, which type of endgame you want to pursue, so you can pick the techs as they come up.