The RFL will want to show the system is working, so expect some grade inflation from C to B and B to A. They may want change, but not upheaval. 16 teams would make their lives easier, but that would need more money -something that looks unlikely in the current climate. So one or maybe (though I doubt it) two C-grade clubs might be up for the chop.

Castleford Tigers - shown resilience, bouncing back from relegations and have a vibrant young team, much of it home-grown. West Yorkshire location means they'll have to look over their shoulders if the new stadium suffers any mishaps.
Catalans Dragons - An expansionist success story. Not only safe, but may pave the way for Toulose.
Celtic Crusaders - Stay in business and marginal improvement on the pitch and they'll get another spin. On target at the moment, but not in a comfort zone. Any thought of further SL expansion into the Celtic countries should be on hold for now.
Harlequins - Only weakness is their crowds - if these stay high enough to keep the business afloat, they'll be shoe in. A second London SL club remains a very distant prospect.
Hull KR - Heading in the right direction albeit from an imperfect position that more than a decade in the lower leagues bequeathed. SL Club of the Year in 2008 and good crowds by C-grade standards. Ground improvements needed and proximity to a rival club guaranteed a place is a two edged sword at best. Team is progressing well (the idea that you can be 'saved' by on-field performance is a weird inversion of sporting principles that I still struggle with). A couple of common sense moves to develop the stadium and some promising youngsters, should be enough - this time.
Huddersfield Giants - Nice ground and improved on-field performance should see them safe.
Salford City Reds - Could do with progress on the stadium and a few hundred more on the gate. On the right side of the pennines at least. With the new stadium, they are safe. Conversely...
Wakefield Trinity Wildcats. - Similar problems to Cas and Salford. Seem to have had one more mis-step on the stadium than the former and lack the geographic advantage of the latter.

So assuming Celtic's medium term viability, it looks like a build-off between Wakefield, Cas and Salford. If they all deliver, Hull KR could conceivably be drawn into it.

The other possibility is that one these or the other SL clubs goes belly up, making the RLF's decision for them - in a few of cases, unfortunately, it isn't impossible to imagine.

Coming up...

Toulouse are the obvious expansion club. Widnes are still the heartlands frontrunners. Those two would be the most likely winners if Salford lost out and could also benefit if Wakefield or Cas went. Halifax are coming up as dark horses, but they'd need it to be a Yorkshire team dropping down, IMO. Cumbria is an appealing idea, but seems unworkable. Leigh might come into contention if Widnes floundered. This might actually be more decided by players than by administrators , given that there is only 1 year after this for any of those teams to reach/win a final that would qualify them for consideration.

If the RFL make the decision half-way through next year we'll say that they didn't look at the evidence and chose their pet team.

If the RFL make the decision at the last minute, armed with all the facts, we will say tht they waited too long and tht the promoted/relegated sides weren't given a chance to adapt.

Any speculation we make this year could be blown out of the water next year, so let's wait until we have all the necessary data to make the decision. If you want to continue the thought experiment "if relegation/promotion happened today" then fine, but the answer to that would hopefully be no change, since the lubs have only just got in and not much has changed since last year.

if they just decide to make it 16 teams, will promotion/relegation resume thereafter?

No , there will never be a return to ' normal ' P and R

If you want it to, then yes. You are talking about hypothesis yeah? This is a fantasy franchise thread - just make what you want up.

Celtic away Catalans last july

I think its a dog fight between Cas, Wakey, Salford, Halifax, Widnes, Leigh, Toulouse and Celtic for maybe 4 spots.

Wigan
Leeds
Saints
Warrington
Catalans
Bradford
Hull FC
Hull KR
Hudds
Harlequins

all guaranteed spots.