I honestly think that Philly, Brooklyn and Dallas are the three most likely to be terrible 2-3 years from now.
Philly
Noel is a defensive specialist with a horrible offensive game, and Okafor is an offensive specialist with a horrible defensive game. Neither can hit a midrange jumper to save their life, so there is no court spacing.
I think Michael-Carter Williams was the piece that gave them a chance. with MCW/Noel/Okafor the 76ers were really only two positions away from having a decent starting lineup - and the bench can always be filled via free agency or trades easily enough. By Trading out MCW they have set their rebuild back a good 1-2 years and pretty much screwed themselves.
The chance of the sixers being competitive any time soon is now basically completely dependant on the recovery of Joel Embiid. If he gets healthy and can make an impact next year, then Noel becomes expendable - they put him on the trading block and hope to get a couple of quality perimeter players back, then start building a roster.
If Embiid doesn't recover then the rebuild gets set back, and things start to get dangerous for the Sixers.
This is officially the third year of Noel's contract, so pretty soon he's going to have an opportunity to walk away and sign with another team if he doesn't want to keep living through the nightmare that is Philly. If an up and coming team (like the Celtics, Bucks, Knicks, Lakers) offer him a lucrative deal, then I think there's a good chance the does exactly that.
This is where the 76ers approach to rebuilding becomes dangerous and risks falling apart. When you remove absolutely every NBA player you have purely with the aim of being the worst team you can possibly be, then it's going to take you minimum 3 or 4 years to get back to relevance. Those draft picks you sign in the first 1-2 years will start reaching the end of their rookie deals by that time, and there's a good chance they walk if you still suck by that point. Nobody WANTS to play for a terrible team.
Brooklyn
The Nets really don't have anything promising. Statistically, the Nets were at their best last year when Deron Williams was on the court - and when he wasn't they were terrible. Now he's gone. Brook Lopez and Theddeus Young are quality players, but neither is a superstar. Also both are 27 years old - not young enough to form the basis of a 4-5 year rebuild.
They don't have cap space, they don't have picks and they don't have young prospects with much upside, so they aren't in any position to speed up that rebuild either.
Thad Young is a good player, but far from a great one. He can't be any more than your 3rd or 4th best player if you hope to be successful. Not unless you have an incredibly deep team, which the Nets certainly do not. That means they have to base their entire franchise around Lopez, a guy who has major deficiencies (e.g. rebounding) and liabilities (health).
The position is pretty bleak for the Nets - if (by some chance) Embiid does come back strong, then the Nets could be a bottom 3 team in the league over the next 3 years.
Mavs
The Mavs are looking pretty bad, but they do have one thing to their benefit and that is Dirk. But if they want to take advantage of that, they need to move quickly.
As long as Dirk is on the Mav's, they are going to be looked at as a team with playoff aspirations. Right now they have a ton of cap space, an owner wiling to spend, and Dirk. That's enough to attract free agents - it was almost enough to attract DeAndre Jordan, after al, and it was enough to bring them Monta Ellis and Wesley Matthews.
Ellis and Matthews are good players, and the still have Parsons too. If the Mavs work hard from now until the start of the 2016/17 season, then they have enough talent on their roster to convince free agents to join their cause. There is an outside chance that they can bring in their next franchise player via free agency and expedite their rebuild. That's really about their only chance at not sucking for years to come. If they can't do that and strike out again (as with DJ) then they could be a pretty bad team.
If they fail at that, then a core of Ellis/Matthews/Chandler is probably not enough to get them to the playoffs - but it's probably enough to prevent them from dropping in to the very bottom of the league. They'll probably be stuck in no mans land, and will need to decide whether to trade Ellis and Matthews (and build around Chandler) or to try to add to the core and stay competitive. Either way it'll be ugly.
Charlotte
Charlotte had terrible ownership (sorry MJ) but they do have a bit of talent on the roster. They won't be a good team any time soon, but I don't think they are bad enough (with Walker, MKG and Jefferson) to be bottom 5 bad. I think they'll be one of those teams who keeps just missing the playoffs (9th - 11th seed).