The Hornets boss is keen to create a team in the summer which can attempt to challenge a place off position and better there 16th placed 2009/10 finish.
Mackay will be keen to land Lansbury on a season's long loan again after the defender made 39 appearances last season for the Vicarage Road club.
Mackay told the Watford Observer: If we can get him (Lansbury) back then I think it will be good for him, good for Arsenal and good for us."
Icelandic striker Helguson will be a tough prize to win after the forward netted 11 goals whilst on loan from Queens Park Rangers, and whilst wages could cause a problem its unclear where he lays in Neil Warnock's plans.
"That is going to be a trickier situation [than Lansbury's] and obviously at a different end of the pay scale as well. It is something that is going to have to be talked about between Heidar and QPR." Mackay added.