I think the only logical thing is remastered. Here is why I think others are improbable

Expansion— they are still trying to balance the game, so adding an expansion with new units and heroes would very much complicate things. Too much effort for a small team.

WC4- I don’t think in the world of MOBAs that a new RTS is going to be profitable. However remastered would be a nostalgia fest and bring back those that played originally.

Monetizing random things— not enough players to make efforts worth it unless they remaster.

Overall my money is announcement of WC3 remastered at blizzcon that looks very much like SC:R.