I suspect people will lean Hyper-Offensive now that Latios is picked, as Latios has the speed, coverage, and insane power to lead a powerful offensive team. To cover its many weaknesses (Bug, Dark, Dragon, and Ice), I propose running Arceus-Steel as a companion for it.
Arceus-Steel @ Iron Plate
252 HP, 4 Special Attack, 252 Speed
The given EV's allow Arceus-Steel to speed tie other Arceus forms at minimum. Thunder Wave is a great move that no Pokemon that is affected by it appreciates; it can increase Arceus's chances at winning a Calm Mind war while crippling offensive-based Pokemon that give Latios trouble, including Choice Scarf Zekrom. Stealth Rock is an option over Calm Mind as Steel is not the greatest attacking type in the game, and Arceus will not often get the opportunity to sweep entire teams (especially without another coverage move). I am assuming this team will use Deoxys-S or another hazard setter though, so SR may not be necessary. Roar is another option, but Thunder Wave can help limit the potential of opposing Calm Mind Pokemon unless they carry Refresh. Even then, both Latias and Arceus-Dark, the most common carriers of Refresh, will struggle with Steelceus as is.
The lack of coverage and immediate power make Steelceus a mediocre offensive presence, whereas the given EV's will not allow it to take super effective moves very well. Fortunately, Latios can cover for Steelceus's Ground, Fire, and Fighting type weaknesses. Note that Latios does not want to take powerful physical moves such as Ho-Oh's Sacred Fire or Terrakion's Close Combat, though it can handle stuff like Arceus-Fighting's Judgment or Scarf Reshiram's Blue Flare pretty well. Despite these shortcomings (and significant opportunity cost), Steelceus would make a good partner for Latios due to its ability to take repeated resisted hits as well as effectively spread paralysis, making it easier for Latios to clean up late-game.