Hi Harko93,
Difficult to advise on an already existing rack, since I don't know your (background) thinking of how you came to this rack, the ideas behind it, etcetera; but I will give it a try:
In my opinion you could do this two ways (or both, perhaps the best!). One way is, adding a nice basic LFO for some simple but good LFO modulation, nothing fancy, just a semi-plain LFO, that can provide you sometimes at least as much fun as a "fancy" LFO. For a good and basic LFO, I can recommend the Doepfer A-145-1 LFO, it's a basic but good LFO and it has parallel outputs for the different wave shapes: pulse, triangle, sine, saw and reverse-saw. It has a switch for speed ranges: low, medium and high that enables you to use this from being a slow LFO to a pretty fast one. This is a reliable LFO that I use in almost all of my patches and I am really happy with it. Naturally you can take any other "basic"/"plain" LFO you prefer, this is just an example.
The other way would be the opposite and get a wild or "fancy" LFO that might give you a lot of more modulation possibilities. If you really want something seriously crazy then I can recommend the Erica Synths - Black Octasource, I call this a psychopath of a module, which it really is :-) In a positive way that is. Because this module is so crazy, you need some time to understand it and tame this beast but once you get it under your control (can you ever have a psychopath under control...? ;-) ) you can really have joyful modules with your dullest modules and you get very interesting results.
Since a basic LFO doesn't need to be too expensive, I would actually advise to take two LFOs, a good basic one, and a "fancy" one, so you can do everything you want from basic stuff to crazy stuff.
Good luck with the choice(s) of LFOs and kind regards, Garfield.