There's so many possible outcomes it's near impossible to predict how things will pan out. I don't beleive it's a case of IAG having to sell slots to another carrier. There will come a point they'll have to release them if they don't use them. Holding on to them while they can makes perfect sense for now, but when crunch time comes they're not likely to operate them for the sake of keeping a foothold in LGW. Not wanting to pre-empt bad news by any means, but if Virgin for example were to fail, BA will do surely what ever it takes to get hold of their LHR slots, which would completely alter their position where LGW is concerned.
Wizz are hell bent on gameplay at the moment. I can honestly say that nobody has ever been as vocal about such a large scale expansion, where normally such extensive commercial terms on volumes amongst all else under strict confidentiality would exist. Their single aircraft base is probably little more than a sign of intent on what they want to acheive at LGW. I'd be surprised if RYR were not watching with the same level of intent, especially as a reaction to Wizz if for nothing else, about growth prospects at LGW while being maybe a little less outspoken about it.
You also have to consider from LGW's perspective, will they really want Wizz as a long-term strategic player? On the whole I'd think so with the likes of Norwegian on very thin ice, the prospect of a BA withdrawal and the loss already of Thomas Cook and Virgin, LGW are not in a position to be fussy, and I don't think low-cost long-haul would be off the table for Wizz eventually. That said, Wizz will no doubt demand a pittance in costs and commercial terms with what the aforementioned operators are, or were previously, willing to pay. Yet again LGW would be well aware of the prospect in possibly losing their business to LTN/STN/SEN at the best, nevermind the worst of times.