If Lelouch R3 is good it will be a miracle. Why?
1. Mecha power creep. Lelouch S1 started with melee/short ranged gun battles and ended with limited flight and beam usage. S2 moved to full arial combat, beam spam, and portable nuclear weapon launchers. It will be extremely difficult to go back in scope and they hit a wall that will be very hard to show advancement from without getting even more ridiculous.
The Guren SEITEN is rolling on the ground in that video, so it seems they're not completely ignoring that technological angle. More broadly speaking, I think it's a matter of coming up with interesting battle scenarios and related choreography sequences (including the animation resources to handle them), which were lacking in most (but not all) of R2's aerial fights. If they can figure out a better approach for this in a sequel, especially if it turns out to be a movie (or a bunch of those), then the structural problem would ultimately be avoided even if we somehow ended up fighting in space, Gundam-style.
2. Fan favourite characters. As shows go on the cast continues to balloon and as the time has gone on, it has clearly become much more difficult for the writers/staff to definitively kill off characters and this is doubly so for characters which are popular. It will be extremely difficult to manage all the various different characters properly whilst introducing whatever new ones are needed.
I think it is a valid point, but a large part of it will depend on what is the true format of this sequel. That will determine what kind of story they want to tell and how much time they have for handling the necessary cast. It's also possible they might go ahead and either disable or isolate some old secondary and tertiary characters early on in order to make space for anyone new. Killing is not necessarily the only possible solution and, at the end of the day, there's arguably less than five or six Code Geass characters whose popularity would be big enough of a factor to prevent them from being removed from the picture in the first place.
3. Retconning/Retreading. Since Lelouch S2 was clearly designed to be the end of the series, they quite quickly tied off most of the loose ends. In order to start this new season they will have to untie those ends or retcon certain things to make t fit. More importantly though, the show completed it's themes, told it's message and ended it's story. To reopen it and keep it as a Code Geass show without retreading on the themes/story beats which have already been completed will be extremely hard since they are still using the same cast with the same character dynamics.
There were still some unresolved mysteries at the end of the story, though I'll admit I was never one of those folks who considered that to be a particularly strong complaint (I didn't need or want to know the origins of Geass, but other people will tell you that not spelling it out was a bad thing). Beyond that, I think some of the character dynamics will have to change as a matter of basic necessity, since there will have to be a new conflict under different circumstances. Besides, Gundam is living proof that it's not impossible to keep coming up with variations and alterations, some more creative than others, of existing themes in order to tell new stories of varying quality.