I reckon when the Tigers finally get back into the Finals, they'll comfortably draw 30K+ to whoever they play. The club has a lot of fans who have been starved of finals football, so even if they drew the Titans or Cowboys, I'd expect them to turn out in big numbers, similar to the Bulldogs last year.
I don't see why we can't be a bit more nuanced with the rules when it comes to Sydney clubs. Honestly, there are really 3 clubs in Sydney who's fan bases are very regionalised and don't travel in big numbers, they being Cronulla, Manly & Penrith. Of those three, the later is getting a new 25K stadium so isn't really a concern in the foreseeable future, however for the likes of Manly & Cronulla you don't want to force them to the likes of Allianz or CommBank if they're drawing teams like the Titans or Cowboys in Week 1.
So why don't we have something like, if it's Sydney v out of town, so long as the ground can hold 18K+ it can host a Final? This means if Dragons want to host a game at WIN, they can do it, and potentially Wests could host one at a revamped Campbelltown, although as I mentioned earlier, I think they'd draw 30K+ when they make it back to finals. Manly would be able to have round 1 games at Brookvale, whilst Cronulla would also be able to host once they get their ground capacity back up to 18K.
For Sydney v Sydney, these need to be played at a ground that holds 25K+.