Around 1900-1902 the French had to draw a S T R A I G H T L I N E I N T H E D E S E R T between the Saharian territories of Algeria (which were ruled from the civilian gouvernment in Algiers, although the territories themselves were military ruled and still under conquest and barely controlled) and the Colonies of West Africa/French Sudan&Senegal which were ruled from Dakar, of course each administration wanted the most territory
The frogs in Dakar wanted the red line, The Frogs in Algiers wanted the Black line, a comitee of frogs tried to come with a compromise (the green line), eventually they settled a border close to the compromise, and thus the Kel Adagh Tuaregs ended up in Mali, where they've been clashing with the Bamako gouvernment for the last 60 years in 3 revolts which displaced hundreds of thousands
Anyway moving onto other plans, "Une mer au Sahara. Mirages de la colonisation. Algérie et Tunisie (1869-1887)" says that the French considered flooding the Chott Melhrir and the Chott Merouane of Algeria and Tunisia, "The final project, as it was presented in 1883, included a lake of 8000 km2 and 24 m deep, at the location of the Melhrir and Merouane chotts, south of Biskra, connected to the sea by a canal of 240 km, 100 m wide and 10m deep." A french Qattara depression in a way, some argued it would have put additional pressure on Tunisia, but that became moot in 1881 when it was made into a protectorate, eventually the project cost prediction grew tenfold and the project was abandonned
And for last, turning into a plan that make the former seem sane, so back in the 1840s, when it became clear Algerians are, in fact, not french, and that a people you conquer and subdue will actually resist, and before a large settler population which needed a cheap workforce came, the administration and army was split into three way to deal with the "Indigenous question": "L'assimilation", "L'extermination" (see bugeaud or Cavaignac's plan, or the underlying ideology behind the gasing of algeria in the 1840s) and "Le Refoulement" (aka deporting arabs either to Tunisia/morocco or sending them in death marches in the sahara), but within the last two movements there was an... interesting... movement, spearheaded by Eugène Bodichon and Eugène Mathieu Subtil inspired by the Ottoman "Sürgün" (forced displacement of ethnic groups, infamously used during the Armenian genocide) to replace Arab Algerian labour with Subsaharian one
"Many French planners found this policy particularly intriguing (Aucapitaine 1859). In it they saw the chance to cut once and for all the Gordian knot of the indigenous question. It was a simple matter to combine the logic of the Ottoman
sürgün with the discourses of extermination and expulsion. Might not the ultimate solution to the problem, they asked, be found the day when they had an Algeria without Algerians?
Some of the various plans involved buying over a hunderd thousand slaves, more than 5,000 a years, from the Trans-saharian trade, who would then be freed and used as auxiliaries in the colonial army, who would then be settled across Algeria, other involved directly using them as slave labour in Algeria.
"Subtil’s 1840 project shows that even as the French state had legally committed itself to ending the trade in African slaves, its highest agents were ready to build an Algerian colony with the labor of people purchased as slaves. More than just tolerating indigenous forms of slavery, in 1840 the French government approved buying slaves, a decision that had it been implemented would have involved the French state directly in the trans-Saharan slave trade and opened the possibility that Algeria might become some sort of slave colony. Rather than a stopgap labor fix, this project hinged upon resolving the indigenous question, a question embedded in the unique condition in Algeria, a settler colony in an already settled land. Its main thrust responded to the sort of ambitious projects military leaders like General Bugeaud envisioned to solve the indigenous question and the security crisis: namely, his military colonies could be built and populated by slaves. "
-Rethinking Abolition in Algeria. Slavery and the "Indigenous Question"
-Apostles of Modernity Saint-Simonians and the Civilizing Mission in Algeria
-Introduction : Un long moment colonial : pour une histoire de l’Algérie au XIXe siècle