Let's be clear then: R2 had a bittersweet ending with more positives than negatives...for the characters themselves. That's why it can be considered happy from an in-universe perspective. Lelouch literally died with a smile on his face and the post-story picture drama(s) present(s) his closest friends and surviving family appreciating his sacrifice to make the world a better place. If you want to talk about a sad ending, the results would be considerably more depressing for them than that. In fact, the cliffhanger ending of the first season was more depressing from almost every other angle. You can criticize the undeserved survival of some, but let's not miss the big picture.
Moving on...I'm willing to bet the ending of Akito will be more somber in tone and content, structurally speaking, even if the protagonist happens to survive. We should all know the setting alone seems to suggest as much, considering the fall of the EU is inevitable and more people are likely to die in battle, proportionally speaking, than during the previous narrative. Which is precisely why a direct comparison would reveal a good number of differences vs. what we already saw.
As for the question of romance...it's still a bit too early to do anything other than throw purely speculative theories around, honestly, since nothing has happened in this direction yet. Not to mention that, if I had to guess, it's more likely only one of his real or potential love interests will live to see the finale...and that would almost certainly be Leila. Of course, we still don't know anything for sure...
