Let's be real we can either go as an ally of another group or we can go CAD and lead the way for a small elite force. So let's talk about the various allied contingent possibilities for the elite of the military.
Grey knights: no brainier right? They bring psychics to the table, can fit their special formation in even when we are point heavy with vehicles and special weapons. They also bring in critical melee for this squad who want to, as the orks put it, chop the shooty. Also they bring in the heavy armor with terminators and paladins. Very versatile and very effective when played correctly.
Blood angels are similar in fassion but more if a shooty option with charging capabilities, the psychic powers an armor can be bought in the form a librarian dreadnaught. And can also provide fast durable tarpit units to infuriate your foes in the form of death company.
Space marines bring a good deal of versatility if you don't want to run valkyres. The stalker hunter formation is a terror of any flying unit and lots of antiarmor can be fielded with just 3 centurians with grav gun/ grav amp.
For fluff you could being the Inquisition, certainly an inquisitor with a squad of death cult assassins supported by 2 priests would be a devastating addition to the tempestus.
My last thought for the day revolves around the ages defence line with an antiair cannon and a vindicare assassin, again this is an anti air grouping, but the assassin can intercept a flyer, then later in the turn and next turn he can use his rifle or pistol due to those weapons not having the interceptor special rule.
I hope this gives more insight into the tempestus. Any thoughts? Suggestions? Other armies that help the militarum's elite? Post it. Let's see more of these guys on the tables