Many times when adding similar games to a game page, it's difficult to pick just one game in a franchise that's similar to a game, when all of the games in a franchise are similar. Plenty of times on a page, I've seen all of the games in a franchise added to the similar list, creating an unnecessarily bloated list.
Even if somebody only added one game from a franchise to the page of a similar game, if all of the games in that franchise are similar to each other, it's likely that they would just add the first game in that franchise instead of the second or third. (i.e. Many first-person shooters may be similar to Halo, but if they're only adding one of the Halo games to the similar list, it's not going to be "Halo Reach" and likely not even "Halo 2", it'll likely be Halo: Combat Evolved) This would likely result in a more bloated similar list on the first game in a series compared to it's sequels.
I think it'd be useful to let wiki editors add franchise pages to the similar lists on game pages, and to perhaps include similar lists on franchise pages as well.
What does everybody think?