I am in the midst of writing a solo adventure as part of my "Sailing the Wine Dark Sea" T&T sourcebook. I wanted to share some of my experiences with solo adventures.
Let me start by saying it is hard work. A lot harder than writing a GM adventure. In writing a solo you must include everything needed to run the adventure. This includes all the GM rulings as to what is anis not possible when the player wants to try something creative. It you don't write it in to the solo, it simply is not there. In a GM adventure if a player asks to dive for cover behind some rocks, the GM can quickly decide if there are suitible rocks at hand and how easy or difficult it is to dive behind them. In a solo if you do not write in a "duck behind the rocks" option it simply isn't possible. There is no fall back on the give and take of roleplaying. This give ant take allows a GM in face to face play to seemlessly work suggestions and ideas from the players into the adventure. As if it wasn't enough to have to think of every possible action and write up results, you also need to write a story that is interesting and entertaining enough to keep the player reading. The adventure also must be balanced to be neither too difficult nor too easy.
Unless it is being written as part of a series you do have the advantage that a solo does not need to maintain any external continuity. This can be a great venue for epic quests with the fate of the world at stake. It is rather daunting to introduce a quest like the Lord of the Rings, destroy this powerful artefact or the world is doomed, into a long running campaign world. If the quest fails you need to start over from scratch. In a solo if the quest fails just flip back to the beging and try again or pick up a different solo.
One popular gimmick in solo adventures that I feel is best used extremely sparingly is the "Sudden Death" paragraph. This a paragraph that without warning kills your character in the middle of the adventure. The set up may be something like this "If you want to search the desk go to Paragraph AA," followed by paragraph AA which reads "You beging riffling through the papers on the desk, saddly the desk is home to a colony of fire ants who devour you alive. The End" Not much fun is that? Better to give at least some clue that the desk might house a danger. This could include giving an option to examine the desk before searching it, thus revealing tiny holes all over the surface of the desk; a description of mouse skeletons on the floor beneath the desk; or a warning given at some other point in the adventure. The player then feels less railroaded. "I didn't recognize the signs" is a lot easier to swallow than "There were no signs." I do have several death pargraphs, but each comes after at least one chance to avoid this fate.
Special bonuses and penalties that apply during the adventure can be fun. Some may apply for the entire adventure: There is a terrible storm raging so missile fire of any kind is impossible. Others may only apply to part of the adventure: After you eat the Red Mushroom you are immune to the first 5 attacks from flame dragons. A unique magical item can make a nice reward for completing a solo. If the character is going to be able to take it with them be sure to limmit it in some way. A "Sword of Victory" which allows the character to always win every combat is not a good reward for a solo. In a long running campaign such an item could easily be stolen, but going from one solo to the next this is not so readilly available an option. An advenure that ends with the character stripped of all their gear will likely prove unpopular, although this can make an effective begining.
Other difficulties of writing a solo include balancing combat encounters, T&T's MR rating can give you an idea of how tough the opposition is. Non-combat encounters based either on the player's thinking and puzzlesolving abilities or SRs on the character's attributes can make a solo more enjoyable for cerebral type characters.
For Sailing the Wind Dark Sea I have several encounters with very tough opposition for single characters, however most of these encounters provide an option to opt out of fighting. A character who doesn't fight will not recieve the rewards of victory, but they will not die either.
As it is quite possible that character's entering the adventure will have a range of equipment (or lack there of) I have provided several oportunities for equipment. In some cases this is specific to a sub-adventure, in other cases it is access to a general market.
In writing this solo I have written each sub-adventure in fairly linear order, and have numbered the esections A, B, C etc with A1, A2 being sub paragraphs of strand A. This makes it easier to keep track of how paragraphs relate to one another. Only after the adventure is complete and thoroughly playtest will I scramble the oder of the paragraphs. I have already found several sections that lack a connection at either the begining or the end, these would be much harder to trace in scrambled paragraphs.
Give some thought to the overall story and what you want to accomplish in the solo. I may yet decide to cut out some of the options have created and use them in other solos. I have a hunt that currently includes 6 different quarry animals plus two additional sub-encounters. This is a huge number of paragraphs, it does however allow for a good replay value. My purpose in writing the solo for Wind Dark Sea is to showcase a number of possible encounters, adventure seeds, bits of the setting, etc for Mythic Greece. Subsequent solos in this same setting will likely be more linear in story.