To make it more engaging for casuals - newcomers, they should to LoL - scheme: you got some mini expansions for free each week, and it changes.
Actually this is a phenomenal idea. It gives casual players a chance to test things out and get used to them and form opinions, and it gives the hardcore players a bit better experience without having to shell out 80 bucks.
I'd also be pretty cool with implementing their RP/IP system where you CAN just grind forever if you really want gameplay elements for free and can resist the shiny little "don't you want them now for a few bucks" button. I'm guessing that's a pipe dream, though.
(For those that do not know, Riot Points (RPs) can only be gained from RL money. Influence Points (IPs) are gained by playing a lot, winning, and so on. You can buy almost anything with IPs, except RP exclusive stuff, none of which are gameplay related, but cosmetic and bragging stuff (and some technical, like porting to another server). Also, there are >100 characters (champions) to play with, which you must unlock with IPs or RPs. Each week, however, ~5-10 characters are free).
Doesn't even need to have a way for you to gain IPs, just give free previews each week, and it WILL get people hoooked. By pro plays a lot of LoL, and while he usually grinds for IPs he did spend some RPs too (not much, but they've been buying each other RPs for birthdays and so). Now, he has few main characters that he plays at a time, but always tries a few free ones, just for kicks. And 80% of his buys are, i think, that week's free champions that he liked a lot.
It makes the game more interesting and not the same all the time, which engages you more, which makes you try new stuff, gets you to like new stuff, and eventually gets you to buy it, cuz you liked it.
But I guess they are already doing this in a different form, by allowing you to play custom games with other people's sets. But you know, just throwing it out there.