DA4 should be set 20-30 years after DAI/Tresspasser

Or they could just tear down the Veil. That would cause some serious changes to occur across Thedas, character deaths, etc.

Yes, I think the geographic move will rectify the 'problems' people have with re-appearing NPCs. That's not to say there won't be any, of course; Dorian is almost certainly going to come back.

Although I don't agree with the OP's position, I have to question your assertion. Why would it make him seem incompetent? He can afford to wait a long time for the right opportunity.

If I were immortal and my enemies were not, the smart thing to do might be to go into hiding and simply wait for them to die.

As other people have said, moving the action to Tevinter removes a lot of the problems concerning world states, since they need have very little influence on the storyline in the north. Of the possible returning characters, only Dorian seems a given, because no matter what choices you made in game, he is back in Tevinter at the end. No one else has to absolutely be included in the action unless the writers choose to do so. Whilst I am one of those who feels that they should not have given the Inquisitor such a personal investment in the Solas storyline if they were not going to bring them back, nevertheless it is possible to have a story set in Tevinter without involving them (except through Dorian's crystal link).

That said, it is going to be very difficult playing a game that does not advance the Solas plot, even if the Tevinter/Qunari war is the main focus of the game. However, for me it would just not work to have such a time break as 20-30 years because it begs the question, what on earth has Solas been doing all this time? The loss of his orb was a setback but I really do not believe that it would take him that long to come up with an alternative. Why are all the elves across southern Thedas leaving to join him if there is no urgency? Where is he keeping them all? If he has a safe location for a large number of elves, why not just let them develop a homeland there?

This comes back to why he feels it is necessary to drop the Veil. There are hints that the Crossroads is breaking down (only certain Inquisitors are able to say this to Morrigan). So much of what the ancient elves created has already been lost, Solas would want to preserve this. The ancient ones have only so long outside of Uthenera before they start aging. Abelas was encouraged to leave the Temple with his sentinals rather than go back to sleep, so in 20-30 years they might well be dead. Not much point in trying to restore their world for them then. Solas seemed to think they would be able to find another purpose with elves such as him. In fact Solas could be aging rapidly in 20-30 years. There was also the question you could pose "Why does this world have to die?", which Solas wouldn't answer because of revealing too much. I suspect it might have something to do with the rediscovery of red lyrium, which is now out and growing on the surface. Since it can grow on any living organism no matter how small and is infected with the Blight, that does not bode well for the future of the world. So there are a number of potential reasons why he might feel a sense of urgency about fulfilling his plan. Plus, why was it necessary to absorb Mythal's power if he wasn't going to use it in the near future?

To have all that climax at the end of Trespasser, including the Inquisitor making a decision about how they wanted to deal with Solas, plus saying (if you disband aggressively) that you are off to save the world again and the map stabbing, would all seem rather pointless if there is then a time lag of 20-30 years during which everyone has either died off or advanced into the senior years.

If Solas was willing to wait why give his orb to Cory instead of waiting for his own power to recharge?

Or kill Mythal. 

1) He wasn't aware that Mythal was still "alive," in a sense. He didn't learn the truth until the Inquisition went to the Temple of Mythal and Flemeth revealed herself to the Inquisitor.

2) Even if we assume he could wait until his own power came back, the Elder One posed a threat to both Solas and Thedas. If Corypheus achieved the godhood he sought or unleashed greater catastrophe by entering the Black City, Solas' plans to tear down the Veil would be foiled. By giving his orb to Corypheus, Solas was killing two birds with one stone: taking out a potential rival and unlocking his orb.

Of course, Solas didn't count on Corypheus being effectively immortal, which is why the plan went tits-up.

I like for the next game to take place shortly after trespasser ends.

I'd prefer it to take place shortly after Trespasser so we can at least tie up those loose ends - Solas' plan to tear down the veil, the Qunari invastion etc. Having the next game be in Tevinter should see most NPCs not appearing again anyway, except for Dorian and maybe a cameo by the Inquisitor if he romanced Dorian or was friends with him. And maybe Fenris hunting down Tevinter slavers.

It would seem odd if Solas was dealt with by someone else, but then we had the Inquisitor deal with Corypheus rather than Hawke, so maybe the new protag will deal with Solas too.