Take a look at the plots given in the manual for the sloper. At the tropical bands, it's nearly omni - but as the frequency rises, a slight dent starts to show, and at 25 mhz, you have distinct lobes in a cloverleaf like pattern.
Now purists will say that we don't have enough information here about mounting, ground conditions, height and so forth - and they're right - but the plots suggest that as a sloper, the higher in frequency you go, the more directional it becomes
This is going to be true for pretty much any wire antenna. As you said, it is impossible to get precise with what to expect because of all of the unknowns here. However, as a general statement the longer the wire in relationship to wavelength the more it will display directional tendencies.
Typically beyond about ½ wavelength straight wires will start to display notches or nulls in the pattern in the cloverleaf pattern Mike mentions above, beyond ¾ wavelength the notches start to become fairly deep. Beyond 1.5 wavelengths these peaks and nulls can become very sharp (the longer the wire, in wavelength, the sharper), and this is a core principal that makes directional wire antennas like the V-beam and Rhombic possible. It also plays heavily in the function of a Beverage antenna.
Remember, this length to directionality function is related to wavelength. So that a 45 foot end fed straight wire antenna might be considered omni-directional at 3 MHz (of course, there is “texture” to the pattern, not truly omni, but pretty close), as it is less than 1/6 wavelength at that frequency, but at 28 MHz it would be over 2.5 wavelength and would have distinct directionality.
With this in mind planning the location and direction of your wire antenna is pretty important. People often think about putting the broadside of a wire antenna in a desired direction, and this works very well for a single band dipole installation, but if they use the antenna for broadband purposes they end up with a null in that desired direction at higher frequencies. For that reason I often lay out wires I intend to be broadbanded with areas of interest about 45 degrees off the end of the wire.
T!