IMO, Wickham didn't have a plan to take Lydia w/ him nor did he have any plan of revenge or extortion. I think Lydia flattered him and was willing to listen to his tales of woe ("I lost money in a card game last night. I'm quite certain that the young Smithers was cheating, but nobody else would back me, so I'm out one month's pay." I can imagine Lydia telling him how wonderful he is, etc., and that it's a shame that he can't do anything about it.)

Wickham may have dropped hints about leaving, and he may have told Lydia that he needed some money to do that -- and then suggested that she should come along and they could be married. She might even have volunteered to go along ("what fun we shall have"). Wickham told Darcy that he never intended to marry Lydia, and I believe that. He intended all along to go to London in order to avoid his debts; he may have thought that he would be able to come up w/ some sort of plan at a later date. Lydia was a willing female AND she probably fed his ego at a time when he was suffering(!) from his lack of success at the gaming tables, inter. al. :-)

Wickham doesn't seem too good at long-term planning -- other than working w/ Mrs Younge to trick Georgiana (which may have been Mrs Younge's plan as much as or more so than it was Wickham's), there is no indication that he plans anything out. He gambles (hoping to get rich?) and he charms young women w/ some money, but nothing more detailed than that, as far as I can see.