What limits high altitude powered paragliders other than the engine? Due to their extremely low wing loading, powered paragliders stall at very low indicated airspeeds compared to other airplanes.  A typical paraglider can continue flying all the way down to 10-15 knots.
That suggests the idea of using a fabric wing for flight at very high altitudes.  At 100,000 feet, for example, an IAS of 10 knots corresponds to a TAS of 85 knots, well below the speed of sound.  Suppose that we take a power source that works at low air density – say, a solar-powered electric motor or an engine with an onboard oxidizer tank – and attach it to a paraglider.  Would anything prevent such an aircraft from flying stably at 100,000 ft?  For example, would flutter be an issue?
Stalling speed increases with altitude. Maximum altitude is the point at which stalling speed rises to meet maximum airspeed. To get higher, the options are to; increase thrust, increase wing area, reduce weight, or improve aerodynamic efficiency. These principles apply to all aircraft using aerodynamic lift.
In the case of the paraglider, its aerodynamic efficiency is not very good. So maximum airspeed is low and the stall speed will soon catch up with it as it climbs. It is worth sacrificing a little weight to greatly improve the wing profile and enclose the pilot and engine - but the plane is then no longer a paraglider.
The other main problem with a simple paraglider is pilot support systems. Time to altitude is measured in hours, during which time the pilot lives inside a pressure suit; the kind of thing worn by Lockheed U-2 pilots on an intercontinental reconnaissance flight. Refreshments, especially drinks, will be necessary. If you put all the support equipment in a great big backpack, like an Apollo astronaut but with longer EVA endurance, where are you going to put the engine? Something a step up from a paraglider again becomes a necessity.