'Cold War' brought back the Ice Warriors after a long absence from our screens and there's a chance to give another long-forgotten enemy a leg-up here. Since ' The Macra Terror' the sentient crabs have got only a mostly forgettable cameo in 'Gridlock' to show for their long wait, which seems on the evidence shown to have lost them a few brain cells. But the Shearsmith Second Doctor could soon have them back on their pincers if the call came for them once more. And with a massive 40 year gap between 'Terror' and 'Gridlock,' a case could be made for saying they've waited far too long.
There are a number of possible ideas for taking them forward. A return visit to the Earth colony they last menaced in 1967 is one there's history there as the Doctor curtailed their plan to mine for gas using human slaves. Or a look at some earlier point of their arrival in New New York before they all succumbed to a collective outbreak of regression could prove interesting. Perhaps it was the Second who drove them underground in a sense, and maybe even helped in the process of turning them back into little more than raging beasts.
Alternatively, they could rise again and lure the moptop to face them. A city in terrified standstill would give both the Doctor and the Macra plenty of space in which to duel, and it raises the prospect of David Tennant making an appearance as the Tenth incarnation of the Doctor to help his younger self out a little in an expansion of the 'adventures with past/future selves' idea. The scope's there for him to respond to a summons much in the same way the 'Destiny Of The Doctor' audios has the Eleventh asking his past selves for a bit of a hand. If they wanted to pay homage to Troughton still further, the Second could send out a hypercube as he does in 'The War Games' with Ten responding some time after 'Gridlock' and pitching up to help, even if it is just to share his updated knowledge of the beasties.
The 'Companion Chronicles' audio series could also benefit from the set-up. Ben, Polly, Jamie and Martha could be left standing or in hiding as their Doctors go off to combat the threat together, and the stories they tell one another to pass the time could have purpose by way of referencing past or future adventures for both incarnations. Would the Second believe that he could go on and turn out as the Tenth has, and could the Tenth possibly believe that he'd ever been as seemingly lackadaisical as his past self in view of the horrors of the Time War? Worth finding out, you'll surely agree.