Oooh, especially if the barbarians could be of a subspecies .... lizardmen for example
the only downside of having the barbarians being subspecies, hybrids, or pure fantasy, is that you would need alot more character models, and possibly armor for them ... talking about yikes!!! for anything other than post-game modding.
Still, I would like for these barbarians, probably human, to have their own unique culture, separate from any of the playable factions. Perhaps allowing these natives to develop, grow, and then forming diplomacy with them could allow you to very niche techs. However, deciding wether the niche tech or the land is more important ... is the interesting strategic decision.
Orrrrr, you could even decide between them giving you the niche tech, or simply serving as manual labor .... or you could have them Bribe you, send you regular payments of gold and other resources, to keep from killing them off. There are many ways to exploit a native tribe that has grown up in your backyard. Perhaps even befriending them and accepting the (arguably useful/useless tech) would greatly increase your prestige.
Example would be a niche tech for Jungle-fighting. You make peace with them, allow them to keep their land, trade with them, and they will allow you jungle-fighting tech for free, as well as the ability to recruit their soldiers if needed (for regular payment of course) ... although depending on how friendly you are with them, these soldiers of theirs could be anywhere from Mercenary sized prices to your own nation's upkeep prices (much lower, for a much stronger friendship).