I did a lot of experimentation on that. One problem with botanicals as a flavouring is getting the measurements right - too much and it really affects the smoking properties. You also want to eliminate the wooden components mostly, so use high quality essential oils or alcoholic extracts. Lavender blossoms and mint leaves, cloves, also hazelnut leaves are an exception, they can be used directly, since they burn cleanly.
Anise oil, lavender oil or extract, mint oil or extract, rose geranium oil, bergamot oil, lemon grass oil are good, sweeten and smoothen the smoke. Patchouli also has a smoothening effect, but adds a wooden aroma, which one may like or not. Orange oil should work, but I did no try that yet. I also did not try clove oil, since cloves burn well (see Kretek) and clove extract can be easily made.
Measurements: You want to stay in the range of 0.1-0.5% ACCUMULATED for all essential oils added. It's tempting to add more, but it only impresses once or twice, it can easily become overwhelming. Vanilla extract can be used in addition (0.1-2%, 0.3% seems just right most of the time).
My favourite recipe is now as follows (I developed it with
Maduro as a base, but it's working equally well with my own stoved Prilep):
For 20g tobacco dissolve 2 drops (=0.5% of tobacco weight) rose geranium oil and 0.3% Vanilla extract in enough good whisky/cognac/rum to moisten all the tobacco (appr. 5ml).
Firmly press the tobacco into a jar with the whisky/rum/cognac.
Let steep at least over night, better for a week, and even better for a month or more. Let slowly dry at room temperature afterwards.
The result has a fantastic room note, which even tobacco haters like, and a really pleasant aroma in the pipe. It does not create additional bite (an issue I found with many other aromatic substances). It also does not affect burn negatively. The aroma is stable till the bottom of the bowl.
You may start from there. If you want to make a rose and lavender tobacco, use 1 drop rose geranium and 1 drop lavender oil instead (so the accumulated amounts are still the same).
Invert sugar can be added (up to 10%) if you find the base tobacco to cigarish (= too low in sugar). Anise oil has a very similar effect anyway. If the tobacco contains already a lot of sugar (like my stoved Prilep), it's actually counterproductive.
Anyway, I believe the key to the sublime qualities of a Lakeland lies in small amounts of high quality botanicals which are given enough time to penetrate the tobacco.