I understand it's hard to make Arbitrations into a perfect gamemode as they're meant to cater for a rather specific audience, and an audience who are rather specific of what they like. There's plenty of ideas from plenty of players, many of which would require deep thought about balancing of many factors. However, i have a suggestion that would improve Arbitrations significantly, and it would be no more than a flip of a switch.
Remove Dark Sector missions from the Arbitration pool.
Now this is quite predictably coming from the fact that all Dark Sectors have Infested as the enemy, and not many players like to fight against them. But even if they were a more engaging faction to fight, there's still a very objective fact as to why DS missions should be excluded:
As this spreadsheet shows, Infested missions are dominating the Arbitration mission pool, encompassing a ridiculous 44% of all Arbitration-eligible missions. This makes it an almost overwhelming chance that any given Arbitration will be on an Infested mission, and i am writing this after having 3 Arbitrations in a row appear on Dark Sectors. No matter how fun or not fun the Infested are to fight, this dulls the experience significantly.
Another problem visible from this spreadsheet is that many of the Infested missions take place on the most outdated tilesets, with a massive bias toward Grineer Asteroid and Galleon. In addition to being some of the weakest tilesets (only Corpus Ship i think is worse than either of those), the fact that Arbitrations are fixed in level means that two missions on the same tileset with the same enemies, are for all practical measures, the same mission. Which means that to further dilute the pool of possibly interesting Arbitration missions, up to 4 different rolls can all be Infested Survival on Grineer Galleon.
If Dark Sectors were to be culled from the pool, it would leave 11 Infested missions for Arbitrations, which would be a perfectly reasonable number, especially for the amount of representation Infested normally get in the game. And don't forget, there are only 5 Corrupted missions too. If you wanted slightly more than that, you could enable Derelict Defense and Survival for Arbitrations; after all, we have to complete those missions to unlock Arbitrations in the first place.
And before anyone brings it up, obviously in a perfect world we wouldn't have boring factions nor tilesets, but i'm purposefully highlighting that culling Dark Sectors would be nothing more than a singular line of code that would significantly improve the Arbitration experience, and a perfectly reasonable change at that.