Any sort of word with the idea of hope/expectation/desire would really do, depending on the context. So, for example, Rackham translates the following of Cicero:
Optare hoc quidem est, non disputare.
This is wishful thinking (lit. to desire, to wish for), not investigation.
Wishful thinking, after all, is just "wishfulness" without logically thinking it out, which is the point of Cicero's contrast.
We see it with spes, too:
Ac primo humani ingenii vitio spei suae indulgens, Abisaren belli socium—et ita convenerat—adventare credebat.
And at first, by that defect of the human mind which indulges wishful thinking, he believed that Abisares, his ally in the war, was coming, for so it had been agreed. (Curtius Rufus 8.14, Trans. Rolfe)
In Seneca's De Tranquilitate, we find the phrase spes inepta:
Canus Iulius, uir in primis magnus, cuius admirationi ne hoc quidem obstat quod nostro saeculo natus est, cum Gaio diu altercatus, postquam abeunti Phalaris ille dixit 'ne forte inepta spe tibi blandiaris, duci te iussi', 'gratias' inquit 'ago, optime princeps.'
Julius Kanus, a man of peculiar greatness, whom even the fact of his having been born in this century does not prevent our admiring, had a long dispute with Gaius, and when as he was going away that Phalaris of a man said to him, "That you may not delude yourself with any foolish hopes, I have ordered you to be executed," he answered, "I thank you, most excellent prince." (trans. Stewart 1900)
Cicero has stulte sperasse, but the context indicates a more malicious reading of spes than "wishful thinking" implies, so that's not really comparable, I think, but Ovid (Fasti 3.686) uses stultam spes to be more "foolish expectation."
illa deum promisso ludit inani,
et stultam dubia spem trahit usque mora.
She tricked the god with empty promises.
And led him on, in foolish hope, with false delays.
Tacitus (Histories 1.12) uses stulta spe to mean "foolish desire," which is perhaps just a bit stronger than "wishful thinking."