Arguably every single series whose fan i've been. Though in many cases i don't have even casual interest anymore. Should be noted that there are only few series i've ever been a fan of.
Civilization series i just kinda... grew distant. It doesn't help every new game keeps missing the high par Sid Meier's Alpha Centauri set. Civs may get individual things better but the overall package keeps staying so far below SMAC in fun gameplay. (And SMAC's setting is so much more interesting as well.)
Still, i keep tabs on stuff. Perhaps someday.
Mass Effect: Mass Effect 3 happened. And i never had high hopes for Andromeda but it managed to trash what little i had. Had it been better, i might have kept up with some stuff, like the novels telling the story of the Andromeda Initiative at least, but as it is, nope.
I can't even bother to replay ME1 and 2 anymore. For all ME1's many flaws, i loved that game, and ME2 i regard (or is it now "regarded"?) as one of the best games ever. But even so, ME3 kinda ruined everything.
Command&Conquer. I wouldn't describe myself as a rabid fan but i liked the series well enough (RA2 aside, never cared for that one). Then EA happened: C&C4 is reason enough. C&C3 had its flaws but i will never understand how someone at EA thought C&C4 would be a good idea. History shows us it was not, for it is a game that killed the franchise. How many games have the dubious honor of being franchise-killers? I would not be surprised if there are other former fans driven away by C&C4.
Red Alert 3 didn't really help either. As i note, i don't care for RA2, and RA3 basically doubled-down on silliness. Don't care for that. Would've liked the seriousness-mixed-with-weird-stuff of original RA.
Halo. This one i'm still a casual fan... at least i keep Halo threads subbed here. Don't participate much though, and i'm not hopeful.
The reason is simple enough: Tonal and stylistical change in the series thanks to 343i.
"External" events played a part though. Xbone reveal was pretty shitty. Poorer specs for higher price (compared to PS4), MS's pushing Kinect and other Xbone features that felt completely unnecessary (not to mention rather US-centric). This lead me to abandon Xbox for a time, and Xbox and Halo are largely synonyms (or were, anyway).
It is not all bad, i did like Halo 4 single player for most part. But Halo 5 basically doubled down on the issues of Halo 4 as i see them. And there is no trace of my beloved Halo multiplayer to be seen.
Frankly, i wonder why do i keep having even casual interest... But perhaps that is because Halo has been one of my favorite games since my early teens. Hard to give up all interest, you know?
Warcraft and Starcraft, and arguably anything by Blizzard. Warcraft has been a pretty big thing for me since... Well, as long as Halo, i guess? Since Warcraft 3. Make that 2004 or so. So many years spent playing the campaign, so many years in the multiplayer. And then World of Warcraft.
Warcraft 3's story was never particularly original. But it was fun, well executed and interesting all the same. Then WoW starts making things weird and retconning stuff, and whatever else.
At the same time, i figured i didn't have anything to do within WoW. I didn't care for Battleground PVP, i wasn't interested in raids and i didn't care for grinding dungeons to gear up. And the world was too structured and small to be interesting to explore (and the aforementioned story idiocy didn't motivate me either).
And between WoW messing with the once-interesting fantasy world even more and Starcraft 2's atrocious writing, i can't bring myself to care about the idea of Warcraft IV, if Blizzard ever will bother with that (Starcraft 2 being a borderline commercial-failure doesn't help).
I will be a casual fan, because Blizzard's games are fun to play, in a way that is rare among video games. But i'll be careful not to become invested.
The Elder Scrolls and Fallout. Morrowind is perhaps my most favorite game ever. And it is unlikely to be topped. I loved Oblivion. And for all FO3 missed points of FO1 and 2, it was incredibly fun. And FONV restored Fallout setting back to Fallout.
But then Skyrim and FO4 happened. I did like Skyrim but as years have gone, its flaws feel more and more strongly. The Elder Scrolls setting is so very interesting, but the game doesn't feel right. Mods can help only so much. The Elder Scrolls Online, while fun, is not really the same thing either.
Unfortunately, it seems Bethesda is doubling down on flawed game mechanics and wrong aspects of their open worlds with Fallout 4. And this is the reason i no longer regard myself a hardcore fan of either series. I fear Elder Scrolls VI won't be an RPG anymore, but rather a merely open world game that becomes a some kind of theme park rather than such a immersive world Morrowind was and is.
Not giving up on either one completely, not yet. But i'm not hopeful.
I don't really become a fan of any games anymore. Too risky. I could live with a series ending at its high point but why does it always have to go so bad first? Leaves a sour taste.