So, if we give Fremen scouts free thumper also, will they be too strong?
I don't see it being a particular problem if Fremen have much better scouting ability and tag more tribal villages (ie by not having their scouts die to worms).
In the long run, I'd probably suggest tweaking the bonuses available from tribal villages (especially if we turn them into botanical testing stations), so that they don't give entire free techs, which is too much randomness and too much power.
However, there is still some potential value in keeping the effect separate from sandrider.
i) You could put thumpers on melee units, but not guardsmen (if guardsmen eventualyl get sandrider, which they'll need if we eventually take away suspensor and thopter access).
ii) You could put thumpers on melee units and not have it on scouts
iii) You could put thumpers on newly built Fremen infantry, so they'll start on newly created units, but not the initial starting scout
iv) there is still some pure flavor value in having a separate thumper promotion, even if it only applies to units that have sandrider promotion. Slightly messier/redundant design, but possibly worth it for flavor value, and explicitly representing thumpers in the mod.
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Entirely separate issue that I am reminded of; you once posted a list of units that barbarians have access to (or rather, the file where this is located). If you post this again, I'll come up with a set of units that I think barbs should have access to.
I think in this could include a barbarian-only "Desert Raider" unit, that was a 1 movement sandrider crysknife fighter, or from a slightly later tech.