We will play Lucas, Gerrard and Meireles as often as we can. If we have wingers we lose the reliance on needing all 3 of them fit all the time, as we can play 4-4-2/4-4-1-1 with Suarez in his "roaming forward" role behind Carroll (he's not really in the hole, but nor is he a second striker). 4-4-2 would make Aquilani surplus to requirements - sell him to get him off the wage bill, and get some cash in hand in addition to his replacement (eg Adam).
This would give us Meireles, Spearing, Shelvey, and Adam as backup CMs, and while none are ball-winners the way Lucas is, I suspect Spearing will be guided into that role (as SG was, I understand). This will leave Adam free to be more of a utility player, particularly in 3-man midfield formations (if we've picked up injuries to say Carroll or Suarez or our wide men), and plan to abuse the cra p out of his passing and set piece delivery. He could be very useful at unlocking 10-man defenses, and I'm sure we will see many more of those next season than this.
I don't think that Parker is as versatile in this regard, but he would probably do well as cover for/alongside Lucas in a more general way, as well as bringing more experience in the role to the squad (though perhaps he doesn't bring anything "new"?). I think either player would be a useful addition, just not in the same way.