Could Airbus resume production of the A380?
The ability to restart production depends on:

is the tooling still available? Aircraft production uses giant jigs for production of the fuselage and wings. Sometimes these are stored when production ends (I've been in a hangar full of stored Airbus jigs), but sometimes they are scrapped.
is production space available? An A380 needs lots of space in the factory. If Airbus can use this floorspace to produce 3 A350s or one A380 per month, the question becomes what is more profitable for Airbus
is the infrastructure still available? The A380 was built at several sites, then assembled at Toulouse. Roads were widened to enable transporting the wings to the factory. Special vehicles were used etc. The French authorities won't start digging up the roads immediately, but in 10 years?

As noted in the comments, the A380 production line is still open, so it's easy to take new orders now. When the line closes, it becomes more difficult.

if the factory has been repurposed for other aircraft, you need to move your existing production to accommodate new A380 orders.
a few years after end of production, practical knowledge starts getting lost. The drawings etc. are still there, but things like optimal assembly sequences, how to move the giant wings in the most efficient manner, have to be rediscovered. When you assemble an aircraft every month for several years, you develop a routine. Restarting production means this routine has to be rebuilt, so the first year or so assembly will take more time and cost more.
when you close the production line, all contracts with suppliers also end. So they start clearing out their A380-specific tooling too. And when you restart, they need to retool and relearn, etc..

So the longer you wait, the higher the cost of restarting production will be. Upgrades can make the aircraft more attractive to customers, but developing the upgrade also costs lots of money ($1B to develop the A320neo), all of which you have to recoup by selling aircraft.
So it all comes down to how many orders you expect to get. A few dozen won't make it worthwhile to restart production once it's been shut down. A few hundred might.
Airbus has already worked on improved A380 variants. In 2015, they were talking about the A380neo:

On 15 June 2015, Reuters reported that Airbus was discussing a stretched version of the A380 with at least six customers. This aircraft, which could also feature new engines, would accommodate an additional fifty passengers. Deliveries to customers were planned for sometime in 2020 or 2021.[293] On 19 July 2015, Airbus CEO Fabrice Brégier stated that the company will build a new version of the A380 featuring new improved wings and new engines.[294] Speculation about the development of a so-called A380neo (&quot;neo&quot; for &quot;new engine option&quot;) had been going on for a few months after earlier press releases in 2014,[295] and in 2015 the company was considering whether to end production of the type prior to 2018[291] or develop a new A380 variant. Later it was revealed that Airbus was looking at both the possibility of a longer A380 in line of the previously planned A380-900[296] and a new engine version, i.e. A380neo. Brégier also revealed that the new variant would be ready to enter service by 2020.[297] The engine would most likely be one of a variety of all-new options from Rolls-Royce, ranging from derivatives of the A350's XWB-84/97 to the future Advance project due at around 2020.[298][299]
On 3 June 2016, Emirates President Tim Clark stated that talks between Emirates and Airbus on the A380neo have &quot;lapsed&quot;.[300] On 12 June 2017, Fabrice Brégier confirmed that Airbus will not launch an A380neo, stating &quot;...there is no business case to do that, this is absolutely clear.&quot;

In 2017, Airbus announced the A380Plus:

The A380Plus, which was announced ahead of the Paris Airshow, will boast a new wing design intended to improve aerodynamics.
Airbus also said the cabin had been optimised to allow up to 80 extra seats &quot;with no compromise on comfort&quot;.
The European plane maker said the new version would burn up to 4% less fuel thanks to its new winglets, which are designed to reduce drag. It will also have an increased maximum take-off weight and need less regular maintenance checks. Airbus said that along with other enhancements, the plane would cut costs for airlines by 13% per seat.

The fact they cancelled the programme in February 2019 indicates the A380Plus didn't get enough interest to justify keeping the production line open.