American has close to 400 NG737's, 600/700/800 series aircraft, on option and if converted to firm orders deliveries might continue through the mid 2010's. That was the deal American signed with Boeing in 1996, the order started with 75 737-823's with options for 425 more, then a few more were converted to firm orders which brought the total number of airframes to over 100 on firm order, however 15 of those were converted for deliveries to Qantas.
-80's are still young but they will in a few years time approach replacement time, so assuming American Airlines gets reorganized and the industry recovers from its crisis, some of the options still held on NG737's will likely be converted to firm orders. The youngest MD
-80, ex-TWA bird, is only three years old but the oldest one nearly turns 20! There are for the moment over 350 MD
-80's at American including those coming from TWA, so it will be a long time before they are all gone and replaced with NG737's. Today, there are lots of MD
-80's in service in American's domestic system but this will change next decade, there'll be fewer MD
-80's and more NG737's around. Remember 20 years ago, there were lots of 727's around in American's domestic route system with the first few MD
-80's showing up: two pilots and two engines, versus three pilots and three engines! The ex-Reno MD
-87's and MD
-90's have already been phased out. So to be clear: you will likely see lots of AA
Super 80's till at least the end of the decade.
I would be very surprised if American doesn't exercise any of the options mentioned above and choose instead the Airbus A32X family, it's unlikely but not impossible. Until 1993, nobody thought that United, a long time Boeing customer, would ever go Airbus. Folks, this is not a Boeing vs Airbus argument, I'm only speculating about American's future fleet.
The Boeing 757's have all been delivered, none are on order. There are I think 122 B757-223's delivered factory fresh from Boeing and 27 B757-231's coming from TWA. Boeing probably delivered 124 75's to American but if you look at the safety record two were lost in disasters. One was lost in Cali Columbia in 1995 and another one was lost 9-11 in Washington DC. The 223's are RR
powered and approved for ETOPS flights but the 231's are PW
powered and not ETOPS qualified. I think that the ex-TWA 757's will be gone before the end of the decade, along with the Fokkers, while American is reducing capacity to where it was in 2000, the ex-TWA 757's and the Fokkers are next to go now that the 727's and ex-Reno birds are gone. American had drastically reduced the number of fleet types.
So to summarize, American's fleet next decade will be as follows:
B737-800 (and maybe 600 and 700)
B757 (RR powered 223 only)
B767 (definitely the 300ER's)
-80 as discussed intensively in this post
By that time, the 767-200's and the A300's will be on their way out.