Honestly, I think the beauty of Bowie x Raj, for me, is the way it can swing between tropes so easily. You've got the obvious popular one—'Raj pining from afar, finally getting the guy'—and it's sweet. But I've seen more lately where Bowie is the one secretly insecure beneath all that perfection, and Raj is the solid rock who sees through it. It's a nice flip. Also, fanon loves to make Raj way more emotionally intelligent than the show lets on, which I dig.
A huge theme is the exploration of performance versus authenticity. Bowie's life is a stage, right? So many fics are about him learning to be 'off-camera' with Raj, who is just... genuinely himself, always. It's a soft, domestic contrast to the glitz, and writers really mine that for cozy, quiet moments that the source material would never have time for. They'll be decorating Raj's room or cooking something disastrous.
Less discussed, but I've seen a few that are downright angsty, playing with the idea of Bowie's career actually destroying the relationship if they ever went public. The pressure, the paparazzi, the 'scandal'—it's heavy but compelling. It's not my usual cup of tea, but the writers who go there really commit to the emotional fallout.