Any other pokemons deserve ex-raid invite?

I think they need to get rid of EX raids entirely, not add to them.

I think they should make all future legendaries available by EX raid only.

There's both ends of the spectrum right there, above

Good question.

I have heard a theory: "All legendaries should have been EX, but back in August they couldn't get it work on time or they simply wanted to cash in more, so they released all but one as regular Tier 5".

Hence, the implication from that is: all legendaries deserve EX. Most of them are extremely powerful (aside from Zapdos/Articuno/Suicune), and some are argued to be overpowered (Groudon, Kyogre).

In the future, Rayquaza could be the next potential EX, which is where my money is on. Neutral DPS higher than Mewtwo, top everything else. Alternatively, they could release "primal form" of Groudon and Kyogre as EX - whose stats are un-nerfed (so talking about 300 Atk and 4800cp). You got a perfect Kyogre? I got a primal form one! Whose weather boosted CP at caught is already 3482!

Mew and Celebi.

Kyogre and Groudon...ops too late.

Arceus.

None!!!! They need to get rid of this bad idea.

Mewtwo is a lost cause... except for the lucky lottery winners, of course ... but others need not be (unless Niantic is stupid enough to make them EX-raid bosses as well :( That might just be too much for many of us.