There are many casual formats possible for Magic, and as long as it is cards in your collection, you can make a format with whatever you want in it. I have been thinking of coming up with a home format that is small and current, and I can find no better format than the M15 frame. It sucks that it begins with Khans of Tarkir, which, although it has something for everyone, including me, ranks low in my list of favorite blocks.
I like Scars of Mirrodin (because of artifacts) and Return to Ravnica (because of gold cards) best, followed by Innistrad because it is our era's Urza block of brokenness and format-dominating cards.
At this point an 'M15 frame' format is as small as the Standard format after Theros rotates out, and that gives me something to look forward to.
The problem I have encountered with every other casual format or cube I have come up with is money.
1. The Artifact Modern blocks: Mirrodin and Scars of Mirrodin. If these packs were cheap I would be building it!
2. The Modern Multicolored cube: Ravnica and Return to Ravnica. I have a ton of RTR, but those Ravnica packs are super expensive... next idea.
3. The Multicolored Supercube, adding Invasion to Ravnica and RTR is an even more costly idea. $10 old packs hits the wallet really hard.
Yes, I could 'go pauper' but let's face it, the rares is where the ridiculously fun play is.
Back to main, I think the M15 frame casual format has great timing and is easy on the wallet. I have already attended pre-releases for M15, Fate Reforged, Dragons of Tarkir, and Origins, and all of these packs are still at Target and Walmart.
If KTK block did not have Treasure Cruise I would have probably not started up on this made-up format, but Treasure Cruise is in this format, and that is a lot of casual fun.