More options for arraying splines and maybe a separate Spline Offset modifier (iToo Software Offset modifier)
With the addition of the new Array modifier some of the inconveniencies of missing this modifier have been mitigated but I feel Array could have a couple of features tailored for Spline class like controlling the bevel types, choosing between end types, automatically dealing with intersecting splines etc. Maybe we need this in the form of separate modifier (called it Outline or Offset modifier) but also giving more power and flexibility to splines within the same Array modifier is to be considered and therefore having at least some of these options in the new Array modifier would be appropriate. Here is an example of how the popular plugin from iToo software - Offset modifier handles all of this: link.
Blend type based by morph similar to C4D
Quoting: "In Cinema 4D, the array modifier has a wonderful "blend" option, with which the array modifier makes copies with a smooth transition from one geometry to another with the same number of polygons. It would be great to have such a function in the element options or add a new option in which you can select another mesh as elements."
Adaptivity in Spline and Surface modes similar to RailClone Adaptive mode
Quoting: "...I'm seeing this behavior where if the spline isn't long enough, it leaves a gap which is to be expected. There should be an adaptivity setting to make it more like railclones adaptive mode where it will adapt the scale of everything if the space available is above a certain percentage of the objects size. i.e. the object will stretch up to 10% before the modifier adds a new object."
Quoting: "In the surface mode when using Face Center mode it would be useful to have a similar adaptive feature there each clone is deformed according to the face vertices. In the example in the image below we have a surface consisting of differently-sized quad faces. The spline objects that are cloned on each face should be stretched so that they connect with each other. Such functionality could provide powerful modelling of complex surfaces where simple surfaces could be turned into detailed surfaces in seconds, similar to the Panelizer MGC by Ivan Maximov (link). With such workflow in mind it would also be useful to be able to mask which Material IDs to generate a clone on.

Links to original individual ideas: