I'd go with both more features and loyalty program.
I'd say more areas to discover would be nice, perhaps a member-only city with NPCs in most buildings instead of traveling cities to find them all? and a Grand Exchange in it as well...
Member-only dungeons so we get access to some monsters in less-crowded areas.
Perhaps some Member-Only boss(es)? It could be a sweet thing to have.
Like adding Nex for example, with Torva armors and such. Anyone with the right stats could wear the new armors, but only members could try their luck beating Nex to get it. It could maybe help the economy as well.
An other idea would be to set the upcoming skills to be Member-Only, for a while at least, it'd be more worth it to be member.
I disagree with the idea to lower the days of the member tickets. Member tickets for 10-15 days of membership? ... Hell. As I donate for mine, I wouldn't really consider donating anymore. I feel like the current pricing is fair, sad thing is Member Tickets are tradeable, it's both a good and bad thing in my opinion. More people can get member, but they're worth less now. An idea to prevent this could be to make them untradeable, except if you pay a NPC like 5m to turn it into a tradeable member ticket, similar to bonds in RuneScape.
Member Tickets into Donator Points? I'd say no. Member/Donator Items are already tradeable, what'd be the point to change a ticket into Donator Points. Just go buy it from other players if you can't donate by yourself.