Hi Sebastior, I've been experimenting with homemade herb mixes for about a year now, here's my two cents:
-Raspberry leaf is the best tobacco substitute in my eyes. It's got a "bold" flavor and a similar consistency to ganj minus the stickiness, which can make it easier or harder to roll into a cigarette. It compacts when you squeeze it, but it's also kind of "slippery" and likes to jump out of the paper. Adds good volume, smokes evenly; I usually use it as a base. Throw in a little (ONLY A LITTLE) lemon balm for some extra acidity; I haven't been able to recreate that astringent tobacco aftertaste yet but lavendar sometimes comes close.
-Mullein, also known as lamb's ear, is relatively tasteless and has the added benefit of helping expectoration (making and moving mucous--you're going to cough!). The consistency makes it kind of difficult to work with; you need to rub it between your fingers to break it up before you can really do anything with it. Mullein doesn't hold itself together that well when squeezed, and it's more slippery than raspberry leaf. Mixes well with anything. Don't bother with the stems, just use the leaves.
-Lavender is absolutely delicious and has a wonderful calming property to it. I used to use it for anxiety, but tolerance to this effect builds pretty dang fast. You don't need to grind the tiny flowers; they usually roll pretty well if you sandwich them between whatever else is in your cig.
-Lemon balm is also delicious and I've experienced a "sharpening" or calm-alert effect from it when smoked alone (dunno if this is supported by any formal testing though) and it has a synergistic effect with lavender (it helps that the two taste wonderful together). It is REALLY REALLY STRONG, so don't use more than a tiny sprinkle in a cig, or a single pinch in an Altoids tin-sized mix.
-Mint. Do I really need to say more?
-Catnip is another mild, easily-rolled herb that has a slight calming effect in some people (again idk if this has been proven).
-Mugwort is purported by some religious/mystical schools of thought to bring on vivid dreams and is used by some people to achieve lucid dreaming. Be aware that mugwort contains thujone which can be harmful in large amounts. Dunno about the safety but I've smoked it both dry and green and it hasn't hurt me; I usually get great dreams when I use it but that's probably the power of the mind over matter. It tastes bitter, not at all gag-worthy but may be too much for some people.
-Motherwort, I've found, tastes like popcorn when smoked. Don't ask me why. It's hard for me to roll, though, cause it came in twig form =/
This last one may interest you more, but BEWARE! This one isn't totally safe.
-Lobelia has a compound in it that is chemically similar to nicotine and will bind to the nicotine receptor. THIS CAN CAUSE A NICOTINE OVERDOSE IF YOU SMOKE TOBACCO OR CONSUME NICOTINE AFTER CONSUMING LOBELIA. IT CAN HURT YOU. I'm not an expert on lobelia and haven't actually tried it myself, for the record, I just figured it deserved a mention. Do your research before trying it out.
Also, hello Bluelight! I've been a lurker for many years, this is my first post ever. I hope my contributions can be of value to our wonderful community =]