Unless I'm mistaken, Mythic dungeons now drop 705 gear at the highest (ignoring warforged rolls) which is on level with world boss Kazzak, Normal HFC Archimonde, and the first couple bosses on Heroic. There's already a really easy and clear progression of gear available to new players. Catch-up mechanics are already in place and they've never been easier. You can level a new toon today and within a week of reasonable play time assuming you have a bit of gold or luck to throw around have yourself geared enough to start running HFC Normal. If you've got a bit more time or gold on your hands you can get geared enough to go right into Heroics, although my entire point is that that's a bad idea. You don't want people gearing up straight for Heroic or Mythic on new toons and new classes because they'll have no skill to raid at that level.
Raid leaders want people to raid Mythic who know the fights on Heroic. Mythic raiders want their mythic level gear to be rare and earned through Mythic raiding. Opening up cross-realm Mythics was a great decision. Adding a bunch of lazy Heroic and Mythic gear catch-up options at the same time wasn't. Nobody pushing mythics wants to see a mythic-ilvl player show up in queue and find out that person has less than a dozen boss kills on Heroic. Ilvl isn't everything, but I can almost guarantee you I could get my ilvl in the 705 range without ever setting foot in HFC. There's already all sorts of "free" gear out there.
By the time you hit lvl 100 you can spend a couple hours in Tanaan jungle group finders and get baleful tokens for all your gear. I did it on a new toon myself recently. Just look for a group named "Apexis Farm". I rolled need on tokens I could use, and after 3hrs I had a 650 token for every single slot (two rolled to 675 lvl upgraded) and about 45k apexis. You can use those apexis to buy the 695 upgrade tokens (20k/ea) for your baleful gear. Drop one on your weapon and you're already better than the 5 of 6 crafted weapon. So if you just want to farm apexis for tokens you can already gear yourself out completely in 695 gear today.
Kazzak drops 5 different pieces of gear all at 705. Most people are running him weekly on all their 100's for felblight anyways, so they most ofl the pieces he drops. He drops bracers, a neck piece, a cape, and two trinkets. One of the trinkets is fun because it has a ton of leech but is worthless for raiding. The other four pieces are very functional. The chipped soul prism trinket is actually one of the two BiS pieces for some classes until they're into Mythics. That's up to 4 very viable 705 pieces available from a world boss who can be bonus rolled.
On top of that, you can buy or craft for yourself three pieces of armor up to ilvl 715 (weapons to 705). Now run some mythic dungeons for 705 gear. You don't need much and you're already easily at an ilvl of 700+.
New toons don't need additional catch-up mechanics.