Morello on Mobility Creep
I think the biggest issue here is that we don't make mobile champions "pay" for mobility enough in their total power budget. Immobile mages are a good example of this fallout, as you mention.
When you have mobile champions that have the same or similar "raw" power output to a champion without mobility, it creates winners and losers really quickly. I agree here.
In this, I think mobile champions should be more tactically flexible (due to mobility) and have less raw power to compensate. I think, though, this will require more and nerfs and buffs. And will mostly be toning down the raw output of mobility champions (as you mentioned, Syndra and co are bullshit and remove gameplay decision-making due to power factors, so buffing immobile mages is not a good plan).
Preseason 2015 actually does help this in a strategy way in the meantime; jungle ganks, on average, are less frequent, giving less total pressure to immobile characters. This is some of what we saw with more traditional mage mids in Season 2 that we think will buff all these characters a little bit in the meantime.