I didn't take the time to read through the thread and it's probably already been mentioned, but dungeons are great SP now and incredibly fun.

You can take a group of 45-49 into the Darkhold, clear all the mobs on the way to the first boss - get some decent loot from the chests, make amazing SP - and it's fun. I haven't personally taken any proper-ranked classes through the R25 dungeon, but I've heard the same stories in there.

They'll be adding more and more dungeons, too, and those will probably always be a viable way of leveling.
Then you have grind parties, which can be fun. SE even said they're adding beast tribe strongholds which will cater to leveling groups. So you'll be able to fight your way through hordes of beastmen inside their strongholds for SP.

...Then you have your normal grinding groups, and leves (which they've admitted are broken 20-30, and are actually decent SP 40+)

There's things to do, and there will be more things to do in time.