What will Niantic do with Regigigas?

Next ex raid boss

honestly a 9% CP nerf will be just fine his stats are similar to Kyogre and Groudon.

Probably ex raid, im also assuming maybe deoxys and arceus could be ex raids

Ideally I think Regigigas should have its attack and defense stats cut for the first 20~30 seconds, then get its act together and return to its regular much higher stats.

It should be treated as all other "uber" legendaries have been. A nerf to it's stats would do just fine.
If it had the 9% nerf applied, it's stats would look something like this:

Atk: 286
Def: 210
Stam: 200

Alot of people get the idea that Regigigas would be overpowered without Slow Start. This is not the case. Regigigas has stats that are on par with other legendaries. It's ability was implemented in the main series games as a way to make it "balanced" for tournament play. The end result is a Pokemon that is gimmicky to use and outclassed by many other "normal" Pokemon.

Niantic has the opportunity to make Regigigas a viable legendary Pokemon, but not if they cripple it. A Regigigas with a moveset of Rock Smash and Focus Blast (like the Regi trio has) would make for a solid fighting type attacker.