Next breakthrough

No idea. With the last 3 months all being a recap of the gen 1 and 2 legendaries it can be anyone's guess. I doubt the Regis would come up till after regigigas comes to raids. The lati twins are being re released now so if it was a legendary I would say kyogre groudon or mewtoo.

To be safe I wouldn't get my last stamp till I was ready to open the breakthrough

If you wait til after a new breakthrough goes live to claim your research box, then yes, you should get whatever we end up getting next. That's assuming, of course, that there isn't a bug when the new breakthrough goes live (like when people claimed their boxes right after Shedinja went live and got Moltres instead).

As for what's next, your guess is as good as mine. Maybe Groudon/Kyogre, maybe another Snorlax-esque breakthrough, or something we're still missing like Kecleon. In other words, it could be something really useful, or it could be dex entry fodder. Heck, if Regigigas ends up being the tier 5 raid boss for March, maybe we'll end up with the Regis in research breakthroughs.

My guess is Lapras with Ice Beam or Ice Shard.

Or they'll just toss the Regi's into the Legendary mix, so you can get one of 11 Legendary's for 2 more months, and people will bombard the YouTubers who seem to have a direct like to Niantic to say "FIX THIS SH**!".

I think we know which one is more likely...

I'm debating doing the same thing depending on what next month's breakthrough is. Hopefully they'll announce it either today or tomorrow at the latest.

If it's a legendary I could see them tossing the Regi trio in for the next three months since they lack raid popularity. If it's a case of Gen 1 legacy edition again I'd love to see Ice Shard/Ice Beam Lapras.

Latios?

Be aware that it’s not local time that governs the change in breakthrough reward. That worked well for me when I was expecting Suicune and got Entei instead. I think the change happens at 1pm pacific on the first of the month but someone else probably has a more definitive answer.