I'd entirely support some references to past FF's but I'd much rather have an original universe than a cobbled-together crossover. Dissidia already did that and even Kingdom Hearts to an extent.

However, In most of the Tales series of games, there's a colosseum where you can fight main characters of previous Tales games. There's no forced narrative or anything, they're just there and it's all good fun. Something like that in FFXIV would be badass in my opinion, going up against Cloud, Cecil, Squall, Kefka etc.