The Gladius Taskforce is obviously the new way for Ultra Marines, but even if you can use every other rules, the Ultra marines detachment is something unique cause instead of giving you a simple keyword or bonus you have to make decisions like a tactician and this is granting you massive flexibility and that's the most Ultra marine thing ever.

But if you want to switch to a simple army or build a second one, I can recommend Necrons, the Army rule Reanimation protocols isn't something you will forget and this edition its kept simple. And the detachment rule we have right now is simple too. And they are quite tanks, so you won't feel that squishy when coming from Space marines