yea, well classic mode came from Dota, which as a custom game from wc3.....but it wasnt the only custom game, there were many many more, castle defense being one of them, and lots of people played the other games and loved them....let me count the ways....
2. line tower wars
3. WINTERMAUL WARS
those are the ones i remember off the top of my head / played
with the birth of dominion, i can image riot following with many more gameplay styles, released over long periods of time, after all, LoL became insanely popular with just the one gameplay mode
i just suggested castle defense cuz it was really fun, and its 10 ppl on the same team instead of normal 5v5