There is clear evidence that Elizabeth would have considered marrying Col. Fitzwilliam if he had shown any serious intentions, once again proving that what she insisted on at the time of engagement was not "true love", but respect and liking, which seems to have been a revolutionary-enough thing for a young woman to ask for at the time!

IMO, one of the Colonel's functions in the novel is to show that Elizabeth is attractive to other worthwhile men besides Darcy, not just bumpkins like Mr. Collins.

BTW, it's not just Elizabeth who thinks well of Col. Fitzwilliam:

In her kind schemes for Elizabeth [Charlotte] sometimes planned her marrying Colonel Fitzwilliam. He was beyond comparison the pleasantest man; he certainly admired her, and his situation in life was most eligible; but, to counterbalance these advantages, Mr. Darcy had considerable patronage in the church, and his cousin could have none at all.

So, even Charlotte thinks that Col. F. is a lot more likeable than Darcy, but to her practical mind, what is a pleasant husband compared to one who can further her own husband's career?

BTW, the line about Col. F's situation in life being "most eligible" makes me think that it's more his expensive tastes than actual lack of money that make him refuse to consider Elizabeth seriously.