It's worth looking at other sports. I was quite taken aback that the Cricket World Cup is actually shrinking in scale over the next decade, from a high of 16 teams in 2003/2007, reducing to 14 in 2011, 12 for 2015 and just 10 in 2019, 8 of whom automatically qualify! For rugby league to have a world cup that is currently on a similar scale to cricket is saying something. Whilst cricket is limited in terms of development outside of the 8 Test nations, it is far more established than RL. For Ireland, Scotland, Netherlands etc. to qualify for a Cricket World Cup is now a big ask.
My personal feeling is to go for a 16-team cup in 2017, whilst retaining the current group format - the only difference being that Group C and Group D don't have the cross-over fixture than was a bone of contention to some in this current tournament. The extra cost of 2 teams during the group stage should hopefully be outweighed by a more commercially attractive world cup.
I agree with IM2 that there is momentum to be harnessed off a quick award. For instance, potential headlines like "Decade of Sport to continue to 2021" etc. but at the same time, there must be a robust bid process in place to ensure that the correct host cities are assessed and identified. At least if it was awarded to "England and France" (for instance) it allows both host nations an appropriate lead in time to get the strategic funding in place, before looking at the localised and operational costs at a later date, presumably after 2017.