As someone who upgraded from a 60D to a 6D, and from third party lenses to L series zooms, my opinion is that bumping the 6D up a price bracket would be a big mistake. Unless Canon maintains a FF body that is price-competitive, they will bleed sales to Nikon and even more to Sony. I had a few good EF mount primes and legacy Zuiko manual focus lenses that couldn't be adapted to Nikon, but these would worked OK on the Sony A7. The size/weight of the A7 was very tempting, it was mainly my dislike of EVF that pushed me to stick with Canon.
The reality of a world with competition is that Canon can't expect to sit still on spec & pricepoint without losing market share. They either increase the spec at the same price point, or maintain the spec and reduce the price point...or they lose market share. The potential to maintain equal profitability while losing market share (by increasing price) is typically limited to companies/products with a significant 'intangible benefit', like the magic fairies that live inside the red dot and improve images in ways that can be neither measured nor articulated;-P. Although Canon has massive brand recognition that brings in sales at the lower end of the market, it doesn't have the 'exclusivity' at the upper end of the market to enable charging a luxury premium for FF bodies - it increasingly has to compete on features & measurable IQ. As long as investment in lenses was a barrier to switching, the loss in market share was mainly restricted to new customers, but now that deserters to Sony can keep using their Canon lenses the trend can only keep accelerating (unless Canon maintains a legitimately price & feature competitive lineup of bodies). Of the 4 friends I have who are avid photographers, three were long-time Canon shooters (with 5DII, 5DIII and 5DII & 6D respectively), and one Nikon shooter. The Nikon shooter has stayed loyal, the 5DII user now has dumped it for an A7s & Leica film body, the 5DIII user has added an A7r, the 5DII and 6D user has sold the 5DII and accidentally dropped and broken the 6D, and replaced it with an A7. None of them have bought any more L-series glass, but all of them have bought Zeiss, Sony/Zeiss and/or Sigma art (including the Nikon shooter).
When it's my time to upgrade in a few years, the FF camera I'm looking for will be at the same or lower price point, but with (in order of importance):
- a flip + tilt screen
- usable AF and MF focus peaking in live view (without having to run magic lantern!) Obviously this would come with no reduction in Ev for the centre focus point (one of the great things about the 6D)
- touch screen to select focus point in live view
It may also have:
- better DR
- better spread of usable AF points (having more doesn't really bother me, but putting some out a bit further where composition guidelines actually suggest we place the subject would be handy).
- pop-up flash
An FF 70D would mostly cover these bases, but a Sony A8 might equally well, and with continued improvement in adapters the barrier of investing in new lenses is minimal.