Now that the first update is under discussion, I thought I'll put together a list of things that, in my experience impact replay-ability the most.
To sum it up in one word, its: difficulty
In detail these were the points I found had a negative impact to this aspect in order of ascending weight:
1. Diplomatic events lend a convenient way to the player to warm up to the AI, and thus later will never declare war. Because these events are interesting and impactful (getting Phidi pops, or giving food to stop starvation), it is hard to simply ignore them for the sake of difficulty, but this also causes the AI personalities to be single sided: always friendly to the player. Bribing Marauder fleets falls somewhat in this same category due to the second point as well
2. AI having issues handling space monsters, pirate and Marauder Raids. This causes them to lose, ships, time and planets they could colonies early-mid game, but are instead left on the map and causing a block for their strategy and easy pickings for the player. In case of Marauders, the AI is having considerable difficulties handling them mostly due to the below points. My guess: making Marauders weaker (only their invasions, the their bases can remain strong so they are not eliminated easily) would probably increase difficulty for the user, as the AI empires would not be crippled so often, and generally grow faster
3. AI having issues amassing large fleets. A strange one: even if abundant money and metal is available to it, the AI does not seem to produce a steady amount of ships. It seems to get over this fact in the late game, so it must be some limiting factor in the early-mid game (maybe a researched tech?)
4. Oval map type missing: This does to only lead to smaller amount "contacts" due to the borders being more condensed but also leads to a lot of empires being "cornered", and in most cases, the human player to have "safe" borders right in the beginning, and the latest after the first conquests. Due to ship ranges, these systems are danger-free and thus defense for them does not need to be considered.
5. AI having issues creating varied fleets. This is mostly probably due to point 3, but have seen it create one or two ship types endlessly with abundant resources when it got to that point which obviously limits the effectiveness of its fleets. It is difficult to tell how much of an impact this one point has on overall difficulty without addressing the above
As diplomacy seems to be a top candidate for the update, I hope that one is green-lighted. Even if the more-advanced features are part of a paid content (such as advanced diplomatic events), I think that increasing the friction between the AI and the player needs to be prioritized as a patch accompanying the update.
There is nothing wrong with having diplomatic requests, but the AI needs to use this method also towards other AIs and build up a preference which can easily put the player in a disadvantageous spot.
e.g.: Who the AI asks for the resource should be dependent on the amount of resource available to other empire and their relationship towards them. Also: it should not be possible to reject a request without penalty. Rejection should either cost influence or relationship penalty and to make this fair, the request should cost influence as well, so the requester cannot "punish" the target empire for free
Other passive diplomatic effects that would make sense from my experience:
- They should automatically dislike empires that their friends dislike (this could be the purpose of the population attitude metric in the diplomacy screen)
- They should automatically dislike empires killing (bombarding) their citizens (considering their main pop, or all the pops that are living in their territory - can be summed up also by the population attitude towards the empire)
- But most importantly: They should dislike other empires that settle systems, they have claimed first (just like a human would do). The more planets are settled this way, the larger the penalty. This would not only naturally lead to friction between war buddies as they snatch away land from a common enemy, but would also utilize the mechanic already in place of having ownership of a system. Its a win-win, fits into the current system (gives outpost a very nice second meaning/function), and would make oh-so much sense