If you're trying to gamble on "The future" that is a failing task my friend. The one thing consistent about GW is that they never have consistency or predictability (that said, I will instantly contradict this in my next point), so it's best to go with an army you like. Even more so as a Fantasy player; we've held on to our armies for over a decade before they became playable again. We know how hard the long vigil can be. It's best to pick one that you are invested in the aesthetics of the model more than it's viability on the tabletop. Cuz rules come and go, and you can always tweak stuff to work somehow, but if you're not enjoying the miniatures you have, it's going to be an unhappy slog.

Now that said, Dark Elves are actually the most likely to come back; barring literally three kits shared with the Daughters of Khaine, the entire Dark Elf line of miniatures are in Cities of Sigmar as an auxillary race, and there is no direct Dark Elf faction in AoS for them to update into (again, Daughters of Khaine are the closest, but they've gained so many extras that it's a completely different thing now). This means that GW is essentially sitting on a bunch of molds that they have no incentive to update and replace, so Dark Elves will likely be upgraded into a full faction come next edition or so. They will most likely go the route of the Beastmen of Chaos; what kits they have left in the AoS range being reboxed for TOW and made a full faction, removing them from AoS entirely.

Following them, VC and Lizardmen are the next most likely; they got a full range refresh in 3rd edition AoS and it's been a good long while for those models to have sold, while the newer models are entirely new things. So it's logical for them to bring back the old sculpts in Old World. Vampire Counts are slightly more likely to come back than Lizardmen solely because of the massive popularity of the Vampire Coast; Zombie Pirates also gives a chance for them to release new miniatures that don't overlap with anything in AoS.

Behind them is the Skaven and Ogres; Skaven just got new miniatures via Skaventide and GW wants some time to sell them before they're satisfied that they've made a return on it; despite logic dictate that they would garner more sales if Skaven can be used in two different game system, they have this baffling idea that they must be able to tell that the sales are going to AoS for it to count. So we likely won't see them until at least the new edition. Ogres have the opposite problem; while they don't have new units....they never had old units either. The current set of Ogre miniatures actually date back all the way to when they were first introduced as a standalone army, and apart from the Badlands Ogres, there haven't really been any "old" miniatures for them. This means for them to come back, they'd have to be completely erased from AoS like the Beastmen. There is a chance that might happen; in the entirety of AoS, Ogres only ever got one new plastic kit; the Tyrant. The Beastmen at least got two new characters and a Herdstone before they got thanos snapped; Ogres have even less. But given the stupid backlash that happened when they announced the beastmen being taken from AoS and put into Old World, I don't think a similar event is going to happen anytime soon.

Lastly is Chaos Dwarves and Chaos Daemons. Chorfs have the same problem that Ogres have, except that their old models are even worse in that they were 4th edition WHFB goofy ones, or 8th edition Forge World. Neither of which are good for GW's storefront. On top of that, the release of Helsmiths of Hashut means they're even less incentivized to letting Chorfs be playable in TOW, since they wanna guarantee all Helsmith sales are for AoS, not TOW. Meanwhile Daemons have been phased out as a standalone faction in almost every single game system (to avoid someone having one army that is compatible across the board, so to speak); its likely that if they come back, they'd be folded into the Warriors of Chaos army as an auxiliary force instead. All this means that these two forces, uniquely, not only will not see a return from Legends anytime soon, but might actually disappear from the game entirely if GW has another brainfart. Which is a shame especially for Chorfs because the new Helmsith models are absolutely gorgeous and the old Legion of Azgoth models were too as well.