RDJ is irreplaceable, it will take several strong characters to fill his boots in the MCU when he is gone. Doctor Strange could be one of them, perhaps he will lead a Defenders-like team, if he doesn't join the Avengers.
Spider-Man should also be a major future character. He is needed for his sense of humor, which the MCU will miss after RDJ. I am sure they are thinking long term with the casting of 20 year old Holland - maybe 17-year Hugh Jackman type long term, if things go as planned.
It's fun to think about - "What is Kevin Feige thinking about?". It all gets revealed to us 2 or 3 years after the path becomes clear to him.
My life has major plot holes.

There have been rumors for a while that Strange is being primed to take a lead role similar to what Stark's has been to date: with Homecoming, he's up to seven of sixteen movies, not counting the brief appearance at the end of The Incredible Hulk.
Since Hulk and Thor have essentially moved on from the Avengers (until Infinity War, anyway), and Hulk, Thor and Strange represent a power level far above the rest of the original Avengers (pre-Scarlet Witch & Vision), a team with the three of them presents a great option for cosmic-level conflict.
If your theater is a space safe from ideas and opinions, you're doing theater wrong.

I doubt his role is that extensive, not nearly what we'll see of Hulk this time, maybe more like the amount that Spidey was featured in Civil War at best. But I would love to see it.
With the success of "Doctor Strange", they will be looking to use him more, ASAP. They might even choose to expand his role in reshoots which I believe would happen soon. If Doctor Strange isn't prevalent in Ragnarok, at least this opens the door for more extensive future associations, and possibly a Defenders-like team.
Team-ups and crossovers are becoming much more the norm than the exception. They know now that ensemble movies can be the big budget spectacles that make the most money, and they'll be looking for ways to expand the ensemble concept beyond Avengers and Guardians. If they had a third team, they could put out a big ensemble movie every first week of May ad infinitum
My life has major plot holes.