Another viable option might be roughly doubling the number of available aid slots from 4/5/6 to 8/9/10.
Doing so might encourage activity amongst newer players and older players alike with the need to find twice as many deals, and accomplishes the same end goal, which is allowing for ~ 2x the amount of aid to be processed.
This is a viable alternative to larger slots. There are some different ways, or a combination of ways this could be done:
Set aid as a function of money reserve. Minimum is 3 mil, but gradually increase the multiplier (as an example, 0.2) as cash reserves exceed a certain value (for example 10 mil). Then a nation that has 10 mil would be able to send 3.6 mil, and 100mil send 9 mil. The caps would be "unlimited" in the sense that you'd need to have unlimited money to send unlimited aid.
Set number of outgoing slots, and possibly incoming slots, as a function of money reserve (1 slot per 50mil, for example). Then a nation with a base of 6 slots and 300mil could send 12 aid packages.
Set number of slots or aid cap as a function of NS. High-NS nations that need cash aid can easily lose 3mil between the couch cushions. As of now, the aid system doesn't serve them at all outside of tech buying.
Moderately increade the aid cap. This is easiest to do, but could be kicking the can down the road as the top-NS bracket keeps moving up, making the increase less and less useful.
Uncap/dramatically raise aid and let the money flow free. This could lead to faster growth for lower-NS nations, obviously with diminishing returns at higher levels. Tech sellers may demand more, but it would be more likely that they just build up out of selling range.
Uncap aid slot usage. The NS-gap could widen as richer nations buy more tech, while the tech market gets a slight boost in profits. "Aid waterfalls" seen after GW1 would return, and alliances with good bank nations could fully rebuild from a war in under a month.