there are a lot of mods that you can activate and play the sp campaign. usually the author of the mod specyfy if the mod is playable in sp/mp or is for mp only. i've played vanilla version of the fl, then i've activated one mod at the time, played it 'till the campaign was over, and found a lot of fun. i found that rebalance 3.5 was, for me, the best and most complex mode. there are more mods, maybe even complex and interesting, but i couldn't test'em all (crappy transfer rate for dwl, or other reasons, not mod's fault). try to read more gamers posts to find recomandations for mods and read carefully the readme of the mods to choose the best for you. have a blast!!!
ps. most of vanilla savegames works just fine in mods, so you don't have to run the game from the begin (but you could lose the fun from the begin). don't forget to delete the "autosave" and "restart" files.

Holy crap there are alot of mod's , has anyone tried "big daddy"?? That one is one of the ones I am considering. The 2 near the top are "calling out to me as well" I also play mostly in single mode(not right at the present moment computer crashed). Kudos to the people who come out with the MODS, must have been alot of time and effort.