At the end of the day, no game, no matter how good is going to maintain your interest indefinitely. There are only so many different ways you can play a game before you start repeating yourself, only so many house rules you can use to before they narrow the experience to an un-fun degree.
The cut-off point will be different for everyone - the sort of challenges & speed runs bakters does sound horribly un-fun to me but for him they put some interest back into the game, whereas I found keeping notes for an AAR helped me maintain a loose sort of narrative during my last few games, even though I never actually wrote them up - but eventually you are going to run out of ways to freshen up your experience. It happens.
If playing is starting to feel like a chore, it's time to move on. Play other games, do other things. You'll probably come back at some point. At some point you may come to the conclusion that you probably won't come back to it again. That's not necessarily a bad thing. I had some great fun with warband, left it & came back to it a few times but eventually I just got tired of the basic fundamental gameplay. I'd rather not ruin the good memories by forcing myself to play a game that I enjoy far less than I used to.
tl/dr : try mods, try multiplayer, try house rules but if that doesn't bring the fun back, don't be afraid to take a break. And as long as you feel like you got your money's worth, don't feel too bad even if you never come back to it.