TLR This is after players have cleared it already, ways that the top guilds can challenge themselves much like the ironman challenge its similar to gimping one's self in order to do something harder.
Less people is basically the first thing that comes to mind, entering Naxx with 20 players and clearing it would be an achievement.. not sure if its possible however.
1-10% mob damage/hp buff i wonder if this would even be possible because you'd want to adjust all the skill damage as well.. but it'd be too much work so perhaps a "players deal 1-10% less damage and take 1-10% more damage"
I also believe players will speed run the raids on their own and likely have timer addons so that blizzard adding one wouldn't be much use.
I do think the player damage taken/reduced output would be the best thing blizzard could do to tune up the classic raids harder.
And i think if the dev's were up for it scaling Karazhan to 60 would be awesome.. and the line i'd personally draw on extra content, anything else would just be scaling down the next expansion of content and by then a server for it would be better, but it would be interesting to see the Wow dev's model Karazhan the way they wanted/would've wanted to do back then perhaps including the entire place.
Now i understand some people don't want to think ahead or have any changes and of course i don't suggest these during the play cycle or even 6 months after their release i'm talking about when most people have done raids already and guilds are looking at timed runs at that stage, but i thought about having a player debuff being the best and easiest thing from a dev point of view to put into classic and should at least tell somebody else so that in 3 years time maybe it gets repeated to the right person.
(I loathe beta's i hate losing progress but i'm so hyped i almost want to play the beta!!)