The 50 over competition will be the first to go this season. It was close to being scrapped ahead of this year any way. Many fans might miss a proper 50 over competition, but given this year was going to see it played as a side event full of kids and reserve players, I'm not sure too many will shed a tear at it being shelved. The question is whether that then gives the ECB the opportunity to keep it permanently shelved.
The County Championship in theory would then be the next to go, although I think they'll really try to keep it some format. Even it that means just playing a handful of games in August/September/October, with threadbare squads due to possible clashing with the 100 and/or t20 (if they even survive). Indeed, it's a form of sport which lends itself very well to radio and TV following, with very easy ability to both play it behind closed doors and provide multiple hours of live programming. That's only one step away from the norm.
The 100 and the t20, together with the England tests, will though be the ECB's main priority. But if we are only looking at August onwards (and who knows), they I struggle to see how there is space to include both. Unless the t20 is significantly streamlined down (eg like the early years - where there was just five group games per county), and/or the t20 is for non-100 players only. So even if the season isn't cancelled entirely - which must be a strong possibility - I can see them delaying the 100 by 12 months. Better to have one competition running as strong and normal as possible, than two which both suffer from restrictions, adaptations etc.