Taxing network externalizes and economic barriers for entry for example. Taxation could be used as natural pressure to correct market failures. Taxing revenue is not what EU countries propose, but it could be viable solution.
Thinking aloud: If company has market share of X% of the population it pays tax from the revenue related to X. If retail company that has 30% market share pays 0.3% extra from its revenue compared to tiny company with just 10,000 customers, it would probably be enough to even out the field and limit barriers to entry.